Welcome to the forum. Fremont merely assembled GM A-body's, it was a new GM assembly plant for 1964 models & built Chevy, Pontiac, Oldsmobile & Buick A-bodys. By the '72 model's, very few Pontiac LeMans & GTO's were built out of the Fremont plant, this was due to Fremont ramping up production of '72 ElCamino's. Most '72 Pontiac A-body production thus came out of Pontiac MI or the Lakewood GA assembly plant. '70-71 & early '72 Fremont Pontiac builds are something I've in depth studied for many years.
Pontiac engine castings were cast in Pontiac MI. Rough castings were machined, then assembled at Pontiac These components were assembled as engines in plant #9
at Pontiac MI. Sequential engine assembly numbers & two letter engine ID codes were also stamped in Engine Plant #9
. There is a good Utube video of these operations. Also a several month series in the early 90's GTOAA Legend titled "Made in Pontiac" gives good insight. Assembled engines were then put in specially made racks & shipped to all the various auto assembly plants by rail car. At the Pontiac MI home assembly plant, engines actually came through an opening in the wall to the assembly line from an adjoining building.
Previously on this forum, there was a post mentioning a Fremont employee's recollection of assembling '64 GTO engines from assembled 1964 389 shortblocks and heads. That particular "recollection" made absolutely no sense for a long list of reasons. One being there was no reason that other versions of '64 Pontiac 389 heads would be in the Fremont auto assembly plant, all '64 GTO engines incorporated the same "716" casting GTO application 389 heads. It also goes against recollections of other previous early Fremont plant employees. The contribution of any stamping to the front of a '64 GTO engine block was limited to the occasion of a 1964 model Pontiac A-body assembly plant employees stamping a W after a 76X or 78X engine code. The X denoted the standard 3spd manual trans application, and when the occasion rose that there was an excess of fully assembled 76X & 78X engines in the plant's engine bank, & 4spd coded engines were needed for impending car builds, the assembly plant worker added the W (for wide ratio Muncie) after the 76X or 76W.