No need to swap chassis to get a 4x4 setup, the original chassis is 4 wheel leaf spring, like most older 4x4s. It would be far easier to swap the axles onto the existing frame/suspension, like the Napco trucks were. There really is no difference in swapping for a 4x4 setup than there would be for a 2wd with IFS.
Also, the wheelbases are not the same '47-'87. '47-'55 1st were 116", '55-'59 is 114", '60-'72 are 115", '73-'87 are 117.5".
