Here is a photo showing pistons available in 1967. If your pistons are original then they are flat top. Notice the valve reliefs are different in the piston tops and your pistons would have the eyebrows in the wrong place for the 670 heads.
If you wanted to continue to use regular gas then the current 8.6:1 compression ratio is about perfect.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
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