When I did my car, the original floor was nonexistent. The Dakota frame has a kick up behind the cab you will need to address. Since I had no floor, the kick up was not an issue to me, but if you have a good floor, that Dakota frame rises up about 8" (maybe more, or less) just behind the cab and remains that additional 8" higher to the back bumper.

Before I started, I parked the Dakota next to the coupe to compare the placement of things.

With both sets of front tires lined up, if I moved the Dakota cab back on the frame 7" (I believe), that placed both firewalls in line with each other.

I moved the Dakota's drive train (V6, 5 speed in my case) back the same distance. I added a 1/4" plate under both motor mounts and simply moved them back. I was also able to move the trans crossmember back. I had to extend the step the trans crossmember sat on back a couple of inches on one side (I don't remember which side) with a piece of angle iron welded in place. That allowed me to drill new holes for the bolts and the crossmember bolted up like normal on the new extension. I also used the Dakota radiator support. I had to move it back the same distance as the drive train. I had to make new ears to mount the radiator support to. My coupe radiator support was also junk. You will probably want to use your coupes rad support, so how you mount it will likely be different then how I did mine.

I also removed the Dakota front frame horns where kick out in front of the front crossmember. I fabricated new front frame horns inline with the rest of the frame rails from flat 1/8" thick metal. The frame horns only have to support the front bumper.

I shortened the Dakota frame just behind where the cab mounts so the wheel base would match. I also had to cut the Dakota rear frame rails off just behind the rear spring hangers.

It sounds like a lot of work, but once the sheet metal is pulled from the Dakota and the coupe, it should be pretty easy to see what needs to be done.

If the back 1/2 of your frame is good, you may want to consider just using the front 1/2 of the Dakota frame. That would avoid the floor issue with the Dakota frame kick up and your cars good floor pan. I did a front clip on a 39 Plymouth (which was the first with like suspension and frames), the Dakota frame slips inside the 39 frame very nicely at about the firewall. If you cut the Dakota behind the trans crossmember, you should be able to remove your frame's X frame support (if your car still has one) and box between the Dakota and the original frame. You would still need to lift off your body, but afterwards you can keep your original floor pan.

Doing a clip is a pretty serious undertaking. Measure 4 times, make sure you have reference points for length, width, center, and height off the floor, on both the new clip and the old back frame, then cut very slowly. Gene