I think I'm gunna take the easy way out and try to take a trip to the junkyard and grab a set of GT40 heads off an explorer or some type of car. Does anyone know which cars had the GT40 heads? I plan on getting them ported once I get my hands on a good set.
The GT40P heads had a revised combustion chamber design, which put the spark plug at a different angle, requiring "P" specific headers for spark plug access. The Mountaineer also had the standard GT40, and GT40P heads.
Check out this site: http://www.stangpro.com/html/articles/gt40p_files/gt40p1.htm for pretty much everything "P" related you ever wanted to know, if you're interested.
It is possible to use standard headers with GT40P heads, however it requires the use of 90 degree boots, and to cut a spark plug socket to access the plug. This picture of the cut plug socket is Urban96's, so I give him the credit for it, and I hope he doesn't mind that I'm borrowing it
They can be hard to find in working condition at the junkyard. And when you do find them they try to overcharge you for them. I found mine through the Arizona Auto Recycler's Association really easily though.
I have a used set of gt40p heads, MAC "P" specific shorty headers, 1.7 Crane roller rockers, and an e-cam for sale if you want em. PM if interested. They were used for maybe 10k-15k miles and then blew a headgasket. I went ahead and did a head swap at the same time while it had to get torn apart. My mechanic said it all looks like it's in good condition, except for one of the header bolts are stripped, which could be fixed for cheap.