The heads would have had to been milled .100" to get the valves even close to the pistons. Of course then nothing else would fit.
You probably have a three way timing set installed at the wrong position.
I just had a customer, two months ago, with similar problem.. Went thru 16 bent exh valves, eight broken studs and 12 pushrods. Not to mention the gasket sets.
I told him the first time what the problem was. But, customer is always right. Hard headed and stubburn too..