Cayn rear callipers can go for as little as $100 and as high in mint condition around $200. I suggest trying wreckers or Gorilla industries if you cannot find them in the classifieds.
Sorry Nik they are totally different. I get asked these sort of questions all the time, due to the brake calliper being used on a 324mm disc the height is far higher than the R32 GTST calliper. Honestly if you're able to fade this brake kit in any SW20 on a race track you're doing well! There just is no need to go bigger.
You are also using the exact same brake pad in this kit as a R34 GTR/STI so there is no real point.