I've been looking around at the Wilwood site and found that if I wanted to keep my 1.25" thick rotors and maintain the current 2.75" (5.94 sq.in) caliper piston diameter, there seems only one caliper that fits the bill, which is a single piston GM replacement-version;
https://www.wilwood.com/Calipers/CaliperProd?itemno=120-5289

I wonder if that GM piston will mount to the stock caliper bracket, even though I would rather have fixed calipers so I could get rid of the heavy stock bracket too.

My current brake system (1973 C-body rotors, calipers & brackets);