Factory compression ratios are way over rated, the 10.75:1 rated GTO YS 350HP 400's are like about 9.8:1 for example. I had no trouble running around on 93 octane Super Unleaded with a 68 428 (030 over with TRW 2282 pistons and #16 heads) and a 69 428 (std bore OEM pistons reringed and #16 heads) , It wont be 10.5:1 unless you deck down the block and mill the heads down. NHRA minimum was 65cc (same as Royal Bobcat tune) and those with thin gaskets would get on up there but dont sweat it man. Also you can take a few CC out relieving the ridge on the intake side in the chambers. Probably around 75cc now if I was guessing some are as big as 78cc on the #62
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