I have GT-40X (X302) aluminum heads and 10-12 boost. I use Motorcraft AGSF-32C gapped at 035. That is the suggested plug for that head according to Ford. It is tapered, 14mm thread size, fully threaded, and has a reach of .708. According to the Autolite site, 104 is the equal Autolite.
Just babbaling here but this comes up a lot.
From what I've read and I might be wrong, the GT40 iron heads can take either a AWSF-32C (Autolite 764) which is not fully threaded or the fully threaded AGSF-32C. The plugs themselves are not the same but the GT40 iron can use either one. If the non fully threaded version was used in a X aluminum head, the non threaded part of the plug could contact the threads in the head before the tapered seat is seated. I think this is where the confusion that comes with what plug to use in the various versions of GT40 heads and different versions of heads on some 4.6L comes from.