i think it is a good idea. if i could have found a deal on a diesel motor/tranny, i woud have gotten one to put in my '59.
if you are worried about the frame, just box it. the diesel motors are heavy, but i doubt that they are too heavy. you will need stiffer springs in the front. how much stiffer i am not sure. you may look and see if 3/4 ton or 1 ton soprings from the same year truck will fit, assuming the axle setup is the same.
drivetrain length issues...well, you will just have to measure it and see if it will work, but i don't forsee and problems.
axle strength, i would throw it together and see if it holds. or you could go ahead and get some heavy duty axles built to handle the torque
