The front end sheetmetal and front core support pose a daunting task.
Question: How do you mate one old body to a completely different newer chassis, while accomodating new mount points for the front fenders, grill, bumper, radiator AND allow enough clearance for a fan?

Answer: You spend a lot of time thinking about it, mocking things up, measuring, cutting and welding. Sometimes you have pure luck on your side, too.
My buddy Marc has been helping pick up where we left off.

Marc dove in and finished up a patch on the inner quarter panel on the driver's side that he had begun when we last worked on the project.

I turned my attention to the front end, and tried to figure out in my head a way of creating some sort of core support to not only support the radiator, but hold up all the front sheetmetal as well.

I had temporarily hung the front sheetmetal and supported it with vice grips clamped to the stubbed frame rails...and the whole front end sheetmetal hung off that pair of vice grips the past 7 months or so.

I wanted to put the hood on to verify that the front sheetmetal was at the correct height in front; in other words, wasn't too low or too high in relation to how the hood would close. I needed that as a baseline before I could mock up what the radiator height would be, which would determine how I fabbed up a core support to hold it all up.

As luck would have it, when we put the hood on, and latched it in the front, I got good other words, the place where I had temporarily secured the front end on the framerails with vice grips was pretty darn close to the overall height they needed to be.

From this, we began to figure out how to make something at this height to hold the radiator and support the front end assembly.
I'll explain this using the pics below:
Here is a shot of how the front frame on the driver's side interferred with the front end sheetmetal, specifically, the grill surround area (the brown metal). The masking tape approximates where I need to remove some more frame for clearance. Keep in mind, up until now, I had only hacked away the front frame sections to be able to put the front end on. No real measurements were made when doing this, I just knew it was a little on the "long" side...meaning I knew I'd need to remove more. Eventually.
Passenger side, looking towards the front. The frame on this side is not touching the front grill surround, unlike the driver's side. The vice grip is holding the pass. side front fender at the correct height, and helping to support the entire front end.
Below: We put the hood on, and checked for fit. it looked pretty good with the fenders temporarily installed with vice grips. It even latched!
Below: Hood gap on passenger side. I must have done something right.
Below: Hood gap front shot.
Next, we mounted up the spalsh pan and bumper, to see what problems we would run into there with those in place:
The worst of it was that the splash pan would have to be notched to accomodate the '64- up early Mustang radiator. The splash pan simply extended inward too far- so far in fact, it was a only 1/4 inch away from the crank pulley:   Go here &  Here for pics.
After taking many measurements, it was determined that the best plan of action was to cut back the stock frame a little more and fab up and weld in block off plates. Then we'd create some brackets to mount up a core support mounted on rubber bushings that would support both the radiator and front end sheetmetal.  Minimal cutting of the splash pan or the grill surround would be needed. The first thing we needed to do was to scribe a cut line, on each frame rail, plumb and reasonably square. (Driver'sside frame rail is below:)
Passenger side frame rail:
Here's our ace with the cutoff wheel, cutting away:
Next, we did the "dirty finger trick" and traced the outline of the opening on some manilla file folder paper. This was cut out, and traced onto a blank of some 3/16 steel plate that would form the end pieces for the frame rails:
Satisfied with the fit after cutting and grinding it into shape, I welded it solid.