I'm putting together my first old school free style bike I ever assembled. I'm not sure what rear brake I should be looking for, that will work for a 1987 GT Performer. It seems pretty obvious that the side pull Odyssey caliper, that I have, won't work. Any suggestions?

They came with a center-pull brake on the bottom. Not one that needed 990 mounts, the Pro Performers came with those in 87, but the Performers used one that just had one mounting bolt. I have a Dia-Compe one in pink but I'm not sure of the exact model, but there are others out there too. Here's what it shows in the catalog.