use GT40's, the same as Cobra GT40 heads, mild improvement over E7's and you don't need to use special headers.
use the GT40Ps, they bump the compression slightly, have better plug location for better combustion and do make a bit more power/flow more than GT40's. You need GT40-P specific headers to use these heads and new plug boots on the wires.
As for other stuff, the upper and lower intake from Explorers are an upgrade from stock Mustang intakes as well. They flow almost identical to Cobra and tubular intakes which cost $100's more, but you do need to configure the EGR to your application.
I can dig up some links on this later, but here is a Mustang site that you should check out for some more/better detailed info: Corral
2000 Ford F350 - 7.3L, 4x4, 6-spd
"It's not leaking oil, it's sweating power!"
1994 Ford Ranger - built 306 SBF project street truck
2006 KTM 525 EXC-Racing - supermoto and dirt bike