I am not an expert, but there are several different versions of the 1966 or "C6OE-6090-_?" cylinder heads. C6OE-6090- can end with C,D,E,F,M,N,T,Y,AA
Stay away from the emissions heads, you would have holes to find plugs for.
There are bumps or obstructions in some or all of them too.
I can't find my resource book that would tell me valve sizes and 'cc' measurements.