That's another plus to using later heads. They come with hardened seats already. Many moons go when I still had my 1968 hardtop, I blew an engine and needed one fast. My local junkyard had a 1969 GTO 400 YS motor. I grabbed it and ran that engine for years. It pinged like crazy but otherwise ran well. I used the best gas I could find, retarded the timing, nothing eliminated the pinging. Eventually I spun a bearing so I removed that engine and took it apart. The valves had receded so far into the heads I could not believe it. That was because after 1973 there was no lead in the gas anymore.

I understand the appeal of a genuine 350 HO, but unless you find a way to reduce compression you will not be happy. What bigchief said about using dished pistons is right on. But that would require a complete engine rebuild costing a lot of money. It is indeed a dilemma. I have a 1968 400 HO engine. I have never used it just because of the compression. I would have to use the low compression heads I have stashed away.

Did you run the engine code? On the front of the block, passenger side, just below the head, is a two letter code. A partial VIN may be there too. The code will tell you exactly what engine that is, what cam, what heads. Use this chart: Engine codes