You need to find out what you have first. If you pull a valve cover and get the casting number off the heads, we can check some of the sites to find out what they are. You don't know if they have been cut down any, do you? Is the motor apart? If you have the heads off, look at the combustion chambers. If they are open type (round, not heart shaped) then it is a later head. Your best bet then would be to get a set of earlier heads. 70 or 71 heads would be the best, unless you can find a good set of 67 430 heads that aren't cracked. 70 heads would be great. What is keeping your compression down at that point is the pistons, I would think. If you're rebuilding, you could go with the 70 heads and the dished 22cc wisecos and get right around where you need to be by decking the block to set the piston top at .005" below deck.
Engine Code is located between #5 & #7 spark plugs stamped on the deck of the block, visible through a relief in the cylinder head casting. After you find the two letter code, go to: http://www.buicks.net/shop/reference/engine_ident_63_75.html to find what block you have.
