Joined: Nov 2016
Here's the thing about GT40's. They are better than all other Ford-made Windsor cast iron heads, except for the GT40P and GT40X heads. The X heads you can throw out because they're an aftermarket performance piece and you're just not likely to find 'em used unless they're thrashed anyway. The P heads are better in every way, except that you get all the exhaust fitment drama.
But the GT40 was only used on Cobras for what, a year? And then on a couple years of Exploder and Mountaineer, and the Ford Lightning. They really aren't that common! People that have them seem to think they are solid gold, and typically sell them for over $500 no matter where you find them.
So, you get something that performs worse than a $300 set of GT40P heads, and spend at least $200 more for the privilege. IMHO, the price gap between that and some Skip White aluminum heads is negligible at that point. Or step up to some Edelbrocks. If your pockets are even deeper, then AFRs or TFS heads.
ANY aluminum head is going to slaughter GT40's in performance. Even the cheapest Chinese aluminum heads will trounce a set of 40's or 40Ps, performance-wise. They're decent for a stock Ford head, but certainly not legendary or anything. And like the 40P's, while they are good for mild to medium performance, once you get over about 350 flywheel horsepower, they start showing their limitations. The reason they offer such snappy performance and economy is because they have smallish ports and really high velocity. Once you push past a certain point, that becomes a liability on the top end.
There are plenty of guys who race with GT40 and GT40P heads, and they're not junk! But it really depends on how far you want to go with them. Realistically, I think the 40 and 40P's can roll right up to the limits of what the stock block can handle anyway. Like with any other project, I think it's important to decide what you want before you start your build, so all the parts can work as a team. Since the P heads are worth about 15-30 horses over the GT40, and have much faster burn characteristics, I think they're worth the spark plug angle drama. It's not like you'd regret buying a good set of FRPP headers anyway, since they work on everything else too. But that's assuming you don't really want to go 'all out'.
If you intend to be faster than 400 horsepower, it would be foolish to waste any money on GT40 or 40P heads, IMHO.
I smile a lot. It makes people wonder what I'm up to...