If you go with the SSBC rear discs, their instructions leave a lot to be desired (unless that's changed?), seeing how the kit is somewhat universal in that it gives you a broad spectrum of adjustment/fitment, just remember to center the caliper's pads/opening on the center of the rotor, I've seen too often some individuals bolt the caliper directly to the axle tube flange bracket supplied in the kit without shimming it in a centered location as it sits on the rotor, this leads to premature inner pad failure, and poor operation....what rear end are you running?, do you plan on removing the thrust button in the 3rd member?

Many other variables to take into account: I generally try and design an ideal 70/30 to a minimum of 60/40 front to rear brake brake bias by choosing a set of rear discs based on what I'm running upfront caliper fluid volume wise as well as piston(s) diameter and pad/rotor surface area so I can avoid running a proportioning valve to limited the rear bias, I generally run the rearward line right off the master, and the front master line I'll "Tee" off for L/R, no distribution block, no proportioning block, generally I make most of my systems and customers systems power assist, running a dual diaphragm booster, if the engines vacuum can support it?, generally a stick car can get by with 8-10" vacuum, an automatic usually needs 15" vac minimum running a dual diaphragm booster to safely have repeated braking ability