[streetstar]

If the new heads you were thinking about had a smaller combustion chamber, it would bump the compression a bit and may cause you to need more octane --- however , that is alleviated somewhat if you use aluminum heads vs. cast iron (you should be able to run 1 numerical CR point higher without detonating vs. an iron head assuming everything else is equal.

So if you have 76cc truck heads now, you may be able to run 64cc alloy heads without a problem