Regarding the 3” versus 3.25” main bearing speed, on a street engine and in many non-endurance race engines it’s pretty much a non-issue.
https://forums.maxperformanceinc.com...7&postcount=12
Smokey Yunick did turn the mains down to 3” from 3.25” to make the 421 SD engines live for NASCAR events. However for street engines it’s not worth worrying about.
There are guys here with 455 engines running deep into the 10s with 600 horsepower street cars driven to the track. I knew a fellow racer running a 455 with 1970 614 RAIV heads in a roadster running in the 8s, shifting it at 7500 and the 3.25” mains weren’t holding him back.
As far as 4.5” strokes are concerned there’s an issue with side loading the piston skirts and the inherently short cylinders in a Pontiac block. A 4.21” or 4.25” crank is a better choice from the standpoint of longevity.