Kent,
GT is a different animal with additional HP penalties. So add the 10 HP for replacement heads and additional HP for GT depending on the combo. And it starts to look like a bad idea on certain combos.
Look at what combo has all of the records in Stock and SS that will answer your question on the year. But that goes out the window as soon as you start going GT in my opinion.
The engine HP factor is a rating, and has nothing to do with the carb that is for that engine. Is the engine factored hard or soft is the name of the game in how far under or competitive it could be.
Dwight's website below is the quickest and easiest way to play with what works and what doesn't.
http://classracerinfo.com/