On these style trucks its mainly the cab mounts, floors and frame mount locations. But also check the frame where the coil spring buckets are and where the rear leaf spring hangers are as well as any cross members. Rear spring hangers and front spring buckets are notourious for pushing apart due to rust between the frame and the piece. Easy solution is to remoce the rivets, remove the bracket and clean up frame. Repair if necessary. Paint everything with POR15 (or your choice of paint) and bolt everything on using 7/16" grade 8 bolts and locking nuts (or lock washers) I guarentee the frame will out last that truck and probably most other trucks as well.

On the body the bottoms of the fenders can get a little rusted out, and the bottom front of the hood also can rust out on these. But if you have seen any signs yet, all I would do is undercoat those areas.