For that sti headgasket you'll want to shave off ~.030" from the piston height. That will reduce the piston dome volume by ~4-5cc. Depending on exact piston/head volumes that will land you in low-mid 8s CR.
They aren't exactly round, so technically turning it on a lathe isn't the best idea. It would probably work fine though. I'd set it on a mill clamped on the wrist pin area. Any decent machine shop could do it easily. If you can't find a place, then send them my way.