"The 307 5A head is not an upgrade from the 3A head, it is worse. The chamber is smaller and it will boost CR. The 4A head has a huge chamber and will significantly lower CR. A #5, #6, #7, or #7a head will flow better and raise cr, try and find a pair of those. "

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"I don't see the early 307s' 5A smogger heads being much different from 3A 350 smogger heads- possibly smaller chambers on the 5A, almost certainly smaller valves. Probably what the feller meant what that #5 heads [-not- "5A"] would flow better.... Yes, the 1968-9 #5 castings make a great choice for performance. Also #6, #7, and 7a [not "7A"]... "
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"Tiny ports, similar to those on the 2A from the 260's. The 5A's will raise compression, the 3A's have 79cc combustion chambers whereas the 5A's have 64cc or something. Yes, I hear they flow better, but does anyone have anything quantifiable on this? Consider upgraded valve springs and rockers while you're in there if you're not gonna change the cam. Heads aren't really a "time being" thing...for me at least. I mean, that's 6 hours out of my day that I could be doing something else. If you're gonna upgrade, upgrade, no use in spending $30 in gaskets twice. Oh...ARP head bolts while you're at it...if you weren't gonna do it anyway... "

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