4 Wheel disc, soft pedal

Yes, kit uses the GM front calipers as rears.

Car is on the floor currently so I'll need to verify later today, but I looked up the kit I bought, got the caliper p/n and did some digging. 2.5" is the bore size according to the almighty Google.
That's a pretty large piston for the rears, what are You using for proportioning?