I agree, the Dakota frame kicks up around 7" to go over the rear axle, and just stays that height.
I have a full Dakota frame under my 48 Plymouth business coupe. I have the 7" higher rear floor & trunk floor. Not a big deal for me because its a business coupe and I have a storage area behind the seat. Had i wanted a rear seat in there, it would have been a big issue.

I'll post an in process build picture of the differences between the front floor pan, and the rear step up floor. The front floor pan is the original Dakota pan from a standard cab. The rear floor is actually the Dakota box floor. Both floor pans are bolted to the regular Dakota body mounts. There was 8" removed from the frame between the gas tank and the rear of the cab. The cab floor was also moved rearward 7". The box floor front was cut off, and the 2nd box floor crossmember is sitting on top of the back of the cab floor. The box floor is bolted to the rear frame using the original center two box mounts. Gene