The GT40 Irons had 65.5cc combustion chambers that will drop your compression ratio from 8.9:1 to 8.75:1. The irons flow a little less than the GT40P's, about 185-190cfm intake, and about 130cfm exhaust.
A solid head, if you pick it up for under $400. Otherwise, getting a new set of GT40P's for $500, then adding the headers is probably the way I would go. Oh, and I'd also have the irons decked for some actual compression.
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