For a bolt on deal:
1978 g-body manual brake master cylinder with a 7/8" bore. Around $80 to $90 for a new one. Smaller bore = more pressure at the caliper. Measure the bore if you buy one at the auto parts store to make sure it is .875" in diameter and not the larger 24mm diameter unit. It does make a difference.
Aftermarket metric calipers with stock bore and piston sizes. Wilwood aluminum is the best option for weight and engineering. US Brake is the lower cost option that uses stock caliper castings. There may be more caliper flex with US Brake calipers since they are using the stock caliper castings. More caliper flex = less clamping force on the rotor. These options elimanate low drag caliper design that came on most cars and truck in the 1980s and 1990s. Do an internet search on "low drag calipers" or "quick take up calipers" or "step bore master cylinder" to understand the issues with using these types of calipers and master cylinders. Low drag calipers require step bore master cylinders. The smallest step bore master cylinder is 24mm, which in my opinion, is too large of a bore for the small diameter of the caliper bore and piston.
Good aftermaket drag race pads. These have good cold clamping properties.
Another set of rear shoes. Replace your short sided shoes already on the rear brakes with the new long ones so you have four long shoes for rear brakes. More shoe area = more frictin area to hold the rear wheel and keep them from spinning.
You already have the rear wheel cylinders from an s10. If you can, remeasure the diameter to make sure they are 7/8" bore instead of 3/4" bore. Auto parts stores are known for selling the smaller diameter wheel cylinders. Larger bore = more clamping forces.
Stainless steel flexible brake lines. This eliminates ballooning that are common in especially old rubber flex lines. Ballooning = less clamping force on the rotor.
This is about all you can do with the stock brake system.
