Oil + additive viscosity

Mattax said

Based on what you posted, the drop in flow is related to something different when the system was switched over to the 360.
FWIW I ran an remote filter for a few years. And even when the oil temps went close to 300*F, did not have the presure drop off with rpm.

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I agree. The 360 can have looser bearings, and maybe more importantly, looser lifter bores, which will cause higher flows. Any restrictions will always show up more readily as pressure drops as the flow increases. I suspect your 318 was just a tighter engine.

I've run remote filters in many cars for 40 years, mostly rally cars (one that revved to 8k RPM regularly) , and have never seen a pressure drop vs RPM like the OP has found.

Drill that adapter out! Some of the older check valves may have come from the day when cartridge filters were still being used. If it were me, I'd try to find a filter with a bigger center hole so that adapter could be replaced with one that is not so restrictive.