The stroke has nothing to do with combustion chamber size, that is determined by the size dome you use. The 795 pistons are needed for the increased rod length (long rod crank) and the cut domes allow room for the extra stroke (you are correct, with a stock cylinder, the pistons will come out the top of the sleeve about 2mm). You have two choices: run a spacer plate to raise the cylinders 2mm or use domes that are recessed to allow the 2mm clearance required. As far as I know, cut domes is preferred over the spacer plate.
The fuel required will be dictated by compression and timing. Basically, with stock timing, you can run up to 160-165ish on premium pump fuel, and with +4 timing you can run 150-155ish on premium pump fuel. If you are interested in HJR, call Kevin and get the run-down from him. He will give you the all the details involved to build one of his ported 4mm engines.
As far as the bill, I agree, it does get expensive...
EDIT: It has been a long day, so if I missed something, someone can correct me...