I dont think a 6bt would fit under the cowl, may be better off putting the shell on a truck chassis.

If you want a weird engine swap that requires a ton of work and gets 30mpg try a honda S2000 motor & trans with a Jackson blower and stick 5.13's in the 8.75, at least it will still be fun to drive still since the motor is light and you can shift it at 9k.

The cummins makes a 1gen Ram drive like a cow on skates, I could only imagine how an A body would drive.