If they're the ones with the 58CC combustion chamber volume, they will have the same compression ratio as the 283 "power Pack" heads, which is about 9.5-10:1, I believe. That's premium fuel range, or at least mid-grade. They should bolt on without a problem, but don't use the 305 head gaskets. The gaskets might overlap the cylinders with the 3 7/8" 283 bore. Here's a handy cross reference chart that gives casting numbers and combustion chamber volumes of a bunch of small block heads.

http://www.enginelabs.com/news/quick-ref-small-block-chevy-cylinder-head-casting-numbers/

Jerry
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