Grigg is right, that five-lug, six-lug combo isn't stock. If its a Chevy the rear has been replaced. Really not a rare thing for an old working truck.
I'd suggest replacing the rear with a 10-lug from a later truck.
The front is harder and you either have a challenge swapping an axle Grigg's way, or a challenge converting to disks, or running old drums.
You might find simply rebuilding the stock front brakes using metalic shoes will do the job just fine if you have a good booster.
