Having been down this road more then once, I might suggest that you keep and use the Dakota's cab floor pan and lower firewall. Both can be trimmed down to match up with whatever body you find, and it makes using the Dakota drive train much easier because everything will still connect. Most of the older stuff (at least around here) usually needs floor pan work. The Dakota floor pan can be moved back on the frame by simply making new body mounts to connect it to the frame. The motor & trans can be moved back the same amount as the cab floor, and then nearly everything on the Dakota can be reused.

I would also recommend you keep the Dakota box floor. Like the cab, it can be shortened and made more narrow and older box sides can be put on top of the Dakota box floor, and you can still use some of the actual box floor mounts. Most older trucks had wood floors that the box side sat on, and the wood is almost always rotted.

All that said, older truck cabs are the easiest things to swap onto a Dakota frame. Most are cheap, and they are pretty plentiful.

30s truck cabs were very narrow, as in you are sitting right beside your passenger, arm to arm.

40s cabs were wider, you get a pair of bucket seats and about 6"-8" between them.

50s cabs got pretty close to the modern era, 3 smaller guys can fit in those (kinda like a Dakota), and the 60s cabs gave you a few inches between the Dakota size seat and the door on each side.

Car widths are about the same as the trucks, except there wasn't a lot of differences between the 40s and the 50s, as far as width.

When I decided to build my coupe, I already had the Dakota truck. I wanted to build a street version of an old dirt track race car. I intended on adding a roll cage to the Dakota frame, so body condition was not a concern. I began looking for a Mopar 2 door car, between about 35 (wide enough to sit comfortably side by side, my wife and I are not small), and a 54. I really didn't like the 55-56 cars, and the 57 & up were too big (long). The body had to be cheap, and its condition was not a concern, but I wanted as complete of a body as I could get (no missing sheet metal).
The 48 Plymouth was the first car to turn up. I bought the 48 Plymouth business coupe for $200, and it was nearly junk (but complete). Everything except the body shell is from a 90 Dakota. There is a build thread over on the Jalopy Journal HAMB board that is still there. The one I did here has fallen off into cyber space somewhere.

I ended up not doing the cage, but the rest of the car came out pretty well, I think. We have had the car on the road for 4 summers now. I'm currently doing a few minor upgrades, but the car has remained largely unchanged, except the cheap paint has faded and looks more authentic.

I'll post a couple of pictures. Gene