Brake advice - master cylinder

[Project D]

Kouki BMC are 7/8" yes, and it should be fine with RX7 calipers.
Also, for BMC with the same bolt pattern as the kouki AE86 you have the MA70 (1" I believe) and the SW20 (7/8" NA and 15/16" on the turbo).

The stock RX7 FC BMC is 7/8" and 15/16" on the FD.

PS: the bigger the bore on the BMC, the harder you have to press on the pedal for the same braking force.
So as long as your pedal don't travel too far, don't go too big.