Replacing FAST EFI on HP 383, with what?

Space Duster, your over Analyzing the whole issue. That cam is also not too big for the heads you have. To be honest as a matter fact, it is a bit small, for the heads.

To effectively use what you have with minimal change, swap to a smaller cam.

TTI has headers that will fit. Schumacher creative services also has tri- Y headers that fit very well.