Look at it the bright way. Does your engine ping there´s an easy way out
With 58cc heads compression can creep up on you really fast.
As you probably know there are two more common sizes when it comes to combustion chambers, and those are 64cc´s (usually performance heads, such as fuelies) and 76cc´s, which are everywhere on 350 equipped vehicles from 73 and up. Some 76cc heads are of a very good casting but ppl in general doesn´t know about them, so they should be cheap
Bear in mind though that some 350 heads also can come with 1.84" intake valves.
Stay away from 624 castings. They are the lightest casting ever. I have no personal experience with them but a very knowledgeable enginebuilder/friend of me avoids them like plague.
/Andreas
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Member of the secret sbc400 club :-)