I had those 4 piston calipers on a 68 Firebird. They too were corroded and worn out. Sent them to SSBC in Clarence New York and had them sleeved with SS.it's not needed. the square-cut o-ring seal fits in a groove in the caliper.
The bad old multi-piston GM calipers had the seal o-rings mounted on the piston. Sliding up and down in the bores wrecked the o-rings early and often. There is an outfit in the mid-west somewhere that will install brass or bronze sleeves in about any caliper, wheel cylinder or master cylinder but it's pointless to do this with the standard GM big single piston calipers. The hydraulic integrity of the caliper does not depend on the condition of the bore. Rather, it's the condition of the piston that counts and they're easily available new.
There is or used to be a company called Vette Brakes or something similar. The specialized in installing stainless steel sleeves in the Vette 4 piston calipers. For decades they were really the only game in town. I believe the guy that started the company died and his children who took it over didn't have the same commitment to quality he did. Many complaints out there about quality and customer service.