If it is truly a "GTO" engine, the heads will be "93s". 92s were on all other 389s except GTO.
The Pontiac used the timing cover for the "front mount" in those days. The aluminum versions of later years still have the bolt provosinos, but I would hesitate to use one as a "mount' in a high-torque engine.
Pontiacs are very sensitive to compression versus fuel quality. Make certain your static ratio is reduced to under 9.5:1.
Comp offers the XE series for the Pontiac. The XE256H is probably what you're looking for. Decent idle, good low- and mid-range power. The "next one up" (XE262H) is the most popular grind we have for "stockers". Works very well in 389s with TriPower and stock exhaust.
My "dream truck" is a '59 GMC (short bed, step-side) with a 505 Pontiac in it, pulling houses down... (:-