Your engine rebuilder should have brought up the gas requirements before the engine rebuild began. Or and least told you afterwards.

Ask him what the part number of the pistons he put in it were. Should be on your receipt.

From the sounds of things, it's a 99% chance the rebuilder used the thick Fel-Pro blue head gaskets that come in the gasket kit which will lower you "stock" compression anyway.

I say run pump 91 gas. If you hear it ping a little time it back a couple of degrees.