The front sounds like it could be original, the rear is either from a newer truck, or an uncommon GMC of about the same age. Does the rear have GMC on the cover?
If replacing axles then the P30 axles with the original 5/10 lugs would be ideal. If you wanted lighter then I have seen a few 8 lug I-beam axles with disc brakes under some step vans, but they are difficult to find. The P-30 with 5/10 lugs are easy to find. The rear axle swap is quite simple. The front axle swap is quite difficult, as it needs to be cut and narrowed to fit under the fenders. Check out the pictures in my webshots album, link below.
Grigg
