They probably installed "Hardened" valves to deal with the unleaded fuel. However, I though the 216 and 235 were heavy duty enough to handle the unleaded gas in stock form!
Also, before you tear the engine out, pour some marvel mystery oil through the spark plug holes and let it sit for a couple days. That could very well free up the engine. Then the previous rebuild would not have been a total waste of time and money.
