Go 72 cc on the heads... I have friend that runs 72 cc heads on his .060 over 455 and says it fine on pump gas.. THats what 11.5 to 12 to 1 compression... I always thought that was a bit much but he says it works for him.. With the 400 you would only be in the mid 10's so that should be great... Id also go with a solid lifter cam to turn up some RPM's ... I know a guy that used to post here and cp ( Hawkeye ) he ran a RA4 headed 400 o the mid 11's with flat solid cam fwiw.
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