The rest of the firewall was welded shut on the driver and passenger side. The Merc and the Lincoln have become "one".
With the firewall attached, I turned my attention to permanently welding the rest of the floor to the body. I started by centering the rear trunk area, side-to-side and even on a diagonal.  I welded temporary supports into the rear of the trunk floor, securing the Merc body to the Lincoln trunk flooring. This will keep the body square and at the desired ride height, while I weld in filler metal around the entire perimeter where the body, where it meets the Lincoln flooring. I supported the rear end of the body at the height I wanted (making sure it was the same side-to-side) by using a couple pieces of conduit that I hammered the ends flat on, and welded it to the trunk area: (vertical bars seen in the pics below:)
With the body square on the Lincoln chassis, I decided to work from front to rear, starting with the door jamb areas. I began welding in plates of 16 GA. sheetmetal, cut to fit, to fill the void between Lincoln and Merc floor areas.
Below is the driver's side lower door opening, welded solid.
After I welded up the floors to the door jamb areas, I wanted to be sure the fenders lined up, and that the doors would still line up and shut after all this cutting/ welding/movement of the body. I put all the sheetmetal back on, and to my amazement, it all did! I must have done something right!
This is a good shot of what things are to come:
After I got the flooring secured permanently to the body, I turned my attention to a few issues that cropped up. The first issue was how to permanently attach the rear body area to the trunk floor. In the pic below, you can see where th Merc body leaves off, and the Lincoln framework lacks mounts for it.
I am still working on a solution to this, but for the heck of it, I wanted to see how the stock Merc bumper would also mount up to the Lincoln frame. To my amazement, the Lincoln frame was only about 1/4 of an inch wider than the brackets still attached to the '51 Merc rear bumper. I spread out the factory Merc bumper brackets a bit, and slipped the bumper into place to get a look:
'88 Lincoln Frame-
------'51 Merc Bumper Bracket
Next, I decided to get going on the steering column, so that if I had to order one, I could keep working on other stuff. I had an old '71 Chevy Chevelle column laying around, so I mocked it up. I was the right length, so the search was on to find a newer GM column that had double "D" order to mate up perfectly with the stock Lincoln steering.

GM steering columns have become the standard in the hot rod/street rod world. All aftermarket wiring kits for re-wiring an entire car are usually based around connections to the GM columns for ignition, turn signals, neutral safety switches, and so on. Going with a GM column will cut through a lot of the hassle when it comes time to rewire the Merc from stem to stern.

Below is the Chevelle column mocked up for placement. Keep in mind, you have to make sure the column doesn't extend to far forward into the seat area. I don't drive with my belly (yet).
Below: Here is a shot of how the column works with the firewall and underdash area. Brackets need to be fabbed to mount the column securely to the underdash area, but the factory Merc cutout in the dash matches the Chevy column perfectly!
I have obtained a thrashed S-10 column and have mocked it up. It slides right on to the stock Lincoln steering shaft, so the hunt is on for a good, solid, TIGHT, S-10 steering column, to be eventually used in the Merc. Go HERE and scroll down for the install. It was fairly straightforward.
Next, I started on the front driver's side floorpan. After I tore the Lincoln apart, I found it wasn't as clean as I thought it was. the driver's side floor pan was Swiss cheese. See above. I found a guy on E-Bay parting out an '88 Lincoln Town Car, and inquired about the condition of his floorpans. He said they were in great shape. I offered him $40.00 to cut out this section of driver's floor, and it arrived a few weeks later. Boy, was he right. It was like NEW! I cut out my rusted original, and seamed in this new floor pan. Here is the before shot, after I cut out all the rusted-out stuff:
Below is a shot of the old, crusty, rusted-out Michigan floorpan, compared to the Washington-State, nice - near virgin floor. No comparison! I got a steal for 40 bucks! It would have taken hours to hammer out a new pan...
Below is a shot of the new floor, and I have it tack-welded into place. I will finish weld this and be done with it. Then it's on to the trunk and rear-wheel wells...
On to the rear wheel-wells...
I couldn't use the stock Merc rear wheel wells because they were so rusted, and I couldn't use the stock Lincoln rear wheel wells because they didn't line up right, so I needed a solution....

Someone from the HAMB alerted me to the fact that boat trailer fenders were available, and I figured that with some work, I could make them serve my purpose:

I bought a set of steel "boat trailer fenders" for a single 15 inch wheel. They were heavy gauge  sheetmetal and set me back about $40.00 for the pair.

I had to split them lengthwise due to the wideness of the opening I was dealing with.
In this shot below, you can see my buddy Jay holding the inward half in position to determine fitment. Looks good.
Another view, looking driver's side - rearward.
This shot (above) shows how much metal needs to be added to the fenders in order to succesfully join the Lincoln floor to the Mercury body. It tapers...from about an 1-1/2" to about 2", front to back.  (Approx.)
After we mocked up the fitment of the new wheelwells, (above) we double checked the centerline of them. The pink string was taped to the body as a sort of plumb-bob to serve as a reference point for the centerline of the rear wheels. The key is to make sure the boat trailer fenderwells are centered in the opening. We double checked our centerline, by measureing the dimensions of the new wheelwell opening in relation to the centerline. Measure the opening and divide this measurement by two, and align it to the string. Your wheel well will be completely centered in the body, and the wheels you use will be centered in the opening.
Here's me, working away....
Here again (above) you can see how much the boat trailer fenders needed to be split to fill the opening. Filler metal will be welded in to fill the gap.
One side of the fenders had a block off, the part that would otherwise be against your boat trailer. This part was faced on the outward side of the quarters, and trimmed to fit them. The inboard side (shown above)  had to be modified a bit to be able to fill in the area between the trunk floor and the new inner wheel wells. The template shown was made from manilla file-folders, and later would be used to cut a replacement from sheet metal.
My rear quarters were toast, so before I could begin replacing the wheelwells, I had to cut out and replace sections of the quarter-panel itself...
Here's what I cut out and got rid of...pop rivets and brazing to repair rust...from the previous owner...shame...
Click here for the next page
Okay, back to the wheelwells. (Below) Here is the template I made for the inner fenderwells. This part will be welded in to the Lincoln flooring.
Here's me, hacking away at the new fenders...
(Below) This is a shot from the driver's side, looking towards the passenger side rear wheelhouse fabbed up.
I positioned the new innermost half in place, and made sure it will fit. I welded tangs on the original Mercury wheelhouse supoorts to accomodate the new ones. (Top of picture, note the screw.)
Here, you can see how the new opening relates to the quarters... in this shot
I have the inner panels clamped up too. They will get welded to the stock Lincoln floor / trunk area.
After I figured out how much metal to add, I started adding the filler strip I spoke of earlier to the split fender.
(Below) Here's what it looks like all welded together...
Here's what it looks like put in place. This side fits well, and is almost ready for final welding. The stock Merc supports that extend from the package tray to the wheelwells will be used. The driver's side is another story because of the Lincoln's stock fuel filler inlet for the gas tank.