Re: 4 piston calipers replacement
O-rings keep water out as well as brake fluid in.
Stainless steel sleeves in these calipers will stop a 27,000 pound WWII bomber as GM used the same design that Girling originated (90 years ago) for the RAF before WWII. Which broke out in 1938 for my dad who went through the whole war in the RAF. He dropped out of Eaton College medical school to enlist were he was assigned to fly with a bomber crew intensely trained for special opps. Anyway disc brake technology was originally used to stop fighter or bomber planes that had higher landing speeds (and where heavier) than WWI wood and canvas bi-wing planes with discs that had Girling calipers bolted onto them. After WWII GI's returning home started hopping up their 1920-36 era cars and speeds soon surpassed the old mechanical drum brakes.
It wasn't until Wilwood started building larger disc rotors for NASCAR that we even had aftermarket disc brakes (with the exception of Jags and Corvettes). Baer and Wilwood are competitors that make a sterling product. Either can be used to upgrade your brake system (with aluminum calipers shedding about 36 pounds on a four wheel disc brake system over cast iron). A newer brake rotor and caliper will be a fine upgrade, but not original.
If you can find a larger diameter rotor your braking (stopping distance before fading) will improve, using your stock four piston calipers. Just keep in mind you must monitor pad run out over the life of the brakes due to the fixed bridge design vs. the full floating single piston brakes. It is part of routine maintenance. Rotors have two dimensions: out side diameter and thickness.
GM dropped the cast iron Girling brake design due to the seal failing allowing water in if you drove through standing water AND cycled the brakes while driving through a puddle. This cause brake failures which resulted in warranty claims which is why Corvettes soon had PBR followed there after with Brembo brake calipers on their initial 11 inch then 11.74 (called a 12 inch), then (12.8" Called a 13 inch followed by the final size so far at 15.5 inch ZO6 carbon fiber brake rotors (which are thicker than the Camaro rotor so a Camaro rotor isn't directly interchangeable with a Vette rotor, but later model Camaros had bigger rotors with the stock thickness rotor).