I am concerned that there won't be enough oil volume or pressure to adequately feed the rocker boxes.
Er ... what "pressure"?
Under normal circumstances, the scavenge side of the pump - which "feed[s] the rocker boxes" - has a greater capacity than the feed side, so the scavenge pumps a mixture of air and oil. How d'you work out there's any "pressure" in that? The oil pump does impart some energy
to the scavenged oil, to push it back to the tank or up the 'T' and pipe to the rockers. But "pressure"?
Likewise "volume". Oil is scavenged from the crankcase. Assuming it isn't leaking out between crankcase and tank/rocker box, how's an oil filter going to affect "volume"?
Owners have been fitting return-line filters to these old clunkers for at least a couple of decades; d'you think there's some sort of internerd conspiracy to hide any "oil volume or pressure" problems from newbies?