I put a 48 Plymouth business coupe on a 90 Dakota frame. I used the entire frame because the coupe's frame was complete junk. (put your hand through the top of the frame rail kind of junk.)

I used a long box, standard cab and had to take 8" out of the Dakota wheel base. I also used the Dakota floor pan and bottom 1/2 of the firewall. When I shifted the "cab" (the floor pan and firewall) towards the rear 7", the firewalls between the 48 Plymouth coupe and the modified Dakota matched up perfectly. I modified the Dakota cab mounts so I could reuse them. I also used the entire Dakota drive train, which I also shifted towards the rear 7" to match up with the "cab" placement. I trimmed the Dakota floor pan and set the coupe body down onto it, built rockers, and attached the Dakota floor pant to the coupe body.

The biggest issue is the Dakota frame kicks up 6" once it gets behind the cab. I used the Dakota box floor as well and I have a step up on the floor, behind the seat. My car is a business coupe without a back seat, so the step up is not an issue, but if you intend on having a back seat, something else would have to be done.

My car has 3 summers of driving on it. The car is a blast to drive (EFI V6, 5 speed, 3:55 limited slip). Gene