Help with rear end (differential)

[Dads 68 GMC]

Can anyone tell me if the complete housing from a 1974 3/4 ton, 4x4 will fit my 1968 or 1971(both 3/4 ton)? Im not sure yet which frame I will use. The 71 has disc brakes and 4.10's and my 68 has 4.56's. This is a complete resto and I would like to be able to drive it more than just around town. Both trucks are 350, 4 speed. The rear end is bad in the 71. It is my farm truck so I will need to fix it regardless. The 74 has 4.10's. If I put the front and rear differential from the 71 into the 68 frame will my drive shafts, etc. be the same? Any help is greatly appreciated. Thanks, Philip

This is all new to me, this is only my second post, I'm amazed at all the knowledge and nice people on this site.