Here we go again.
Probably a solid bounce-press with somewhere between 325 and 385. Still a good press for a guy who wasn't ever more than, say, 225.
He had muscle, but he was never huge. Muscle is not size or mass.
As well, developed pecs aren't a good indicator of superior pressing strength. If it were, Arnold would have been a huge bencher. He wasn't, relatively speaking. At his very strongest, early-to-mid fours.
Top shelf strength is much more about nervous system conditioning and training specificity.