The first thing you should remember is that back then a 1/2 ton and 3/4 ton are two very different animals. Just look at the different spring packs used in each truck. Half ton springs are 1-3/4" wide. I think yours are wider on the 3/4 ton. Also, the overall lengths are most likely different. A 1/2 ton frame rails are tappered like an a frame. A 3/4 ton frame is only tappered at the front. To use a 1/2 ton front crossmember you will want to make sure that the spring pins are in the same location as on your 3/4 ton and I would use the 6 lug disc brake kit and not the 5 lug. As for the rear, put jack stands under the axle and take off both tires. Measure the face of one brake drum to the face of the other brake drum. This is the measurement you will need when looking for a new axle. Spring purches can easily be cut off and new ones welded on. Hope this helps you out.
