Ford 5.0L windsors most certainly did have the GT40P heads installed,
The 1998 EL GT (only 250 produced) and NL Fairlane by Tickford (only 106 Produced) were the first to see these heads and it is my understanding that they were the only ones to do so???
I dont know of them even ever being fitted to the 351w block as they are somewhat different??? I am not really sure..... Maybe I am wrong, I have never heard of them being fitted to the 351w block???
Do a web search on the "Fairlane by Tickford" and you will find allot of information about these heads as I did. Some of which is a very interesting read.
I myself own an NL Fairlane by Tickford #075 and they are basically all GT interior and drive line from the EL GT transplanted into the Fairlane Ghia bodyshell. Also known as a GT Limosine!
Hope this is of some help to you?