Engine 258 7/16 or 1/2 head bolts by wrench size

[2ndCJ7]

I'm looking for a 1/2 head bolt AMC 258 i6 / 4.2l block. Is there any definitive way to tell without pulling a bolt? Is the wrench size (not thread size) different on the head bolts. I know a 7/16 should be 11/16" or 5/8" and 1/2 should be 3/4 in the rest of the bolt world. I'm just looking for a quick way to find out which block I'm dealing with on CL or marketplace.

 

[Manic Mechanic]

I think by year of manufacturing. It was just the early to mid 80s that had this smaller head bolt