the Chevytalk forum has several discussions on this. As I understand it, the later model axles can be swapped out but are a little wider. Backspacing on the rims can help. The spring perches may need to be changed. Another plus is that you can get disc brakes on the front. The GMC trucks about 63 had hydralic clutch and its harder to get the the disc brakes set up. But this is better than trying to make the frame fit the body...
