There are several companies out there that can bore yours out and re-sleeve with a stainless steel insert. It then uses std.
components. Had this done to mine several years back and it works great. If I recall, it was SSB in New York that did mine.
You have 2 choices - rebuilt cores with steel sleeves. By now I doubt there are any original cores left.
There are also new calipers; I have heard a good rebuilt caliper is the way to go.
Lonestar is the supplier to most parts houses.
There are 2 piston tryes: lip seal (original design) and o-ring.
The o-ring style is considered the better design.
The hardest part of 65 thru 82 Corvette brake work is bleeding.
I use a Motive Bleeder.