You might try emulating the Ford Mod motor three valve head. The bore is only slightly greater than the small bore of a slant six (3.552 for the Ford, 3.4 for the slant) and guys are commonly pushing the bore of the slant six out to 3.5. .
That would be a great-breathing head for sure, but I wonder about the bore center spacing. That might be identical, or nearly identical with the /6, OR it might be way off... that would need to be addressed.
