You don't really need a proportioning valve. You can get 1/2 ton or 3/4 ton calipers, from a 70's era or a 80's era. The rotor you need is for a 78 3/4 ton, I think it's # 2533 at autozone, $25 a piece. For calipers, I think the 1/2 ton and 3/4 ton have different size pistons, which will change your braking power. If you get the same size pistons that you have in the front, then your rear braking will be equal to the front. If you use the 1/2 ton ones, it will be a little less, which may negate the need for a proportioning valve. If you have a 10mm line that you can scrounge up, get 82 calipers, if you have a 7/16 line, get 78 calipers.
I wouldn't spend $150 on drum parts, cause it'll cost you that much to do discs. I think my total cost was $154.99
You can take your calipers off with a 3/8" allen wrench. Just remove the two caliper bolts and take them off. Rotors in the front are similar to the rear hub. Take out the hub lock nut, removing the lock ring, remove second hub nut, then take the hub and rotor off as an assembly. Pound out the studs to separate rotor from hub.
Anything I haven't mentioned can be found in search.