The area in red will be rebuilt, about 1/2 inch outward to clear the frame.
Here is a good shot showing what was cut away and needs to be replaced.
The inner rockers must be rebuilt first, before the body can be permanently welded to the flooring. Otherwise, I'll never be able to get to them once the body is welded to the floor. This was time consuming. I used 16 GA sheetmetal since this area is structual. I cut out all the old stuff, trying to find "good" weldable metal. I read once in one of Tex Smith's customizing books: "make it about twice as strong as you think it should be, and it should come out about right" I did...
Here is the finished driver's side boxed lower a-pillar. At this point, the rocker has new metal welded in on the backside, the corner is inset about a 1/2 inch to clear the frame, and the outer skin of the rocker is screwed in place temporarily.
Here is the backside of the driver's side rocker, viewed from inside the car, looking through where the floor will be.
This is a view of the passenger side rocker, looking at the newly boxed a-pillar from the engine-rearward, and new metal welded in to form the back of the rocker panel.
Another view of the passenger side boxed a-pillar.
Next I began welding the front firewall to the Lincoln chassis...(below) this point, I have the body centered from side-to-side, front-to-rear, to assure it's square on the chassis. I measered from several points, over and over, until it was the same on the chassis from side to side.

I welded the corner plates (the triangle areas below) in first, and then welded in some temporary supports in the rear, extending from the trunk opening of the Merc body to the new Lincoln trunk floor, to keep the rearend square where I wanted it. Note: when I rebuilt the inner rockers and moved them outboard, I now have almost an inch  of clearance between the frame rails and the inner rockers...
Here is the finished product, in place over the chassis. Note the clearance.
And here is the passenger side inner rocker to frame rail detail...
Below: Here is shot of how the driver's side inner-floor area looks. I decided to mount the body at the stock Lincoln floor pan height. I started welding at this point, rather than channeling the body lower than the frame, because of two things.

1) It would be easier to lower the suspension to obtain the "look" I was going for, rather than try to raise it up to be driveable, and

2) it would be easier to weld in the floor of the Lincoln level with the old Merc flooring rather than have to make it step down.
Here is an overall side shot showing the mocked up rockers, fenders, and quarters at the as-welded height.
Here is a shot of the relation of the front wheel to the fender opening at this ride height. I think by the time I add in the weight of the front end sheetmetal to the chassis, the radiator, etc, it will come out at about a driver height. Airbags come later.
Below- for what it's worth, these rockers are cheap ($60 bucks) Taiwanese knock offs. THEY SUCK. I don't know what they were intended to be used for, but it sure wasn't a '51 Merc 2 door. I had to bend the begeezus out of them to get the contour right...and I'm still not happy.