mill the heads run 13:1s roller. Id go back and tell them you want your 396 back.LOL!you could have had a four bolt block.
stock 396 with 10:25 was a 325 horse motor and they ran pretty good on 91 octane. may have to back timing down a few degrees but I dought it. My 396/375 horse ran all the time on premium unlead (91) and I only had a problem once and that was when I ranway over 38 deg. total advance racing it and burnt a hole in a piston that time.