I haven't gotten that far yet. Still in the planning stages.
So far I have been running my Slant on ethanol free pump gas, which is 90 octane here. I can get 93 octane easily, which is actually a tiny bit cheaper, but it is "up to 10%" ethanol gas.
There are variations in castings, so the chamber diameters vary, it may take slightly different amounts to reach a specific volume. Basically a stock 225's pistons are ~.180" in the hole, & the compression w/a factory shim gasket is lucky to be 8:1.
"At Std. Bore Dia." every .010" removed will reduce volume a tick over 1.5cc's.
Have fun...