Yup. Your best "bet" is to stick with Ames and PY. Bill knows his Pontiacs, too.
Our "main" business is the 400 "resto-motor". A close second are the 461 "strokers". It's easy enough to get the 350-plus HP "back" from dropping the compression, and IMO, a GTO should ALWAYS run like a GTO...(:-
There is the phenomina known as "inaudible detonation". Many engineers SWEAR it doesn't exist. Maybe. Maybe not. I know I've torn down dozens of 389s and 400s where the owner said the same thing, not a sound. But the rings fell off in pieces and some of the pistons were burned badly at the land. Contrary to popular belief, you can't "cheat" by retarding the timing. IF "spark knock" is your problem, of course, changing the timng WILL help. Detonation is another animal entirely. Nothing short of adequate octane will "cure" detonation.
Not saying yours is afflicted, just be "on gaurd".
Welcome to the high-class muscle car crowd! (GTO didn't earn its reputation from LOSING....)