You might be able to convert to rear wheel disk brakes from a Camaro. The rear ends are similar(both are GM 10 bolts), but do have some differences. It might be easier to check aftermarket companies for a conversion kit.
There is a kit for the G-bodies(monte carlo,cutlass,regal) that might work for the front. I know the 1st gen S-10's use the same spindle as the G-body cars. If you just want better braking, try updating your stock brakes. I would use slotted rotors and some good metalic pads. Avoid the cross-drilled rotors. They are prone to cracking, especially on daily driven vehicles, plus they are really expensive. You could also get metalic shoes for the rear drum brakes. Why do you want bigger brakes? Do you race the truck at all? Good luck and let me know what you find out. I have an 85 S-10 X-cab that could use better brakes-- especially when towing(or racing!!). Later
I would avoid cross drilled unless they are forged with the holes there. i know that the setup from www.movit.de are not drilled, but they are forged inthe rotors themselves. expect to drop a pretty penny on them though.
are you wanting to have disks on the back? i only ask why do that? what reason do you have to want to upgrade to disks.
one thing to remember is that with the trucks and most of the gm vehices and all other vehicles most of your braking occurs with the front brakes. usually anywhere from 75% - 95% depending on the vehicle itself. if you do any i suggest the front brakes.
go with a larger rotor, braided lines and better pads if you want to do it right. if you are on a limited budget i would upgrade your pads and lines atleast. make sure that your rotors on the front are clean and not warped.
One could obtain a prportioning valve and have their brake system setup to use more braking power in the rear. I think your percentages are off as well. I believe it would be more like 60% cuz typically rear drums should be replaced every third time you replace disks.
I'm pretty sure the front brakes do perform about 90% of your braking. When you hit the brakes, most of your vehicle weight shifts to the front wheels. The reason drum brakes last longer is because the return springs pull the brake shoes in so they don't make contact with the drums. Disc brakes are always touching the rotors, even when you let off the brake pedal. Plus with the weight shift when braking, the front disc brakes have to apply more force on the rotors to stop the truck.
If you replaced the proportioning valve with an adjustable one and raised the brake bias to the rear, you will be locking up the rear wheels under normal braking. I know this because I had a proportioning valve go bad and I was always locking the rear wheels. After I replaced it, I disassembled the old one and found that the part of the valve for the rear brakes was sticking open and allowing too much pressure to the rear brakes. It wasn't stuck completely open, but it was enough to cause problems. Later.