Is 9.7:1 compression on iron heads too much for 91 octane

I hope these are not KB199's as those are for a Ford 427 LOL. Probably intends KB167...

Actually the 399 domes pistons will run in a closed chamber head. But the OP will be well off with the flat tops. SCR of around 8.8 with the stock 318 head (675's) and DCR in the low 7's. Change to 62 or 63 closed chambers (or mill .030" off the present heads) and the DCR will be in the upper 7's. Pretty easy to run there with pump premium.