At the risk of sounding like a broken record, I will repeat my recommendation of those (formerly) cheap ol' Edmunds heads. They are unlike any others in that they have the spark plug moved juuust enough to miss the exhaust valve. This allows the use of extended tip spark plugs that seem to be an advantage. I put a set on the '51 Merc engine in my '51 Ford in 2016, and they haven't missed a beat yet. I pulled the plugs last summer to check their condition and they still looked great. I'm a true believer.
Here's my original thread :
https://www.fordbarn.com/forum/showt...hlight=Edmunds.