Be careful assuming the pressure drop is coming from the oil cooler. I run 2 oil coolers in series with a std melling pump at 60psi and no pressure drop both on a 331 and 347.
However, on separate occasions throughout the years I have confirmed 5psi pressure drop using regular aftermarket oil filter vs. K&N, 5psi from an oil pump with the relief spring cap pressed in only 90% of the way, and another 5psi from 10W-40 vs. 15W-50 oil weight. I have also found 5psi from from tired, but undamaged bearings at high rpm (oil slings out of the slightly wider bearing clearance at high rpm and the pump can't keep up with the volume, so pressure sags a little)
One way to be sure the cooler is or isn't the restriction is buy a smaller cooler, then plumb it in parallel with the first one. Put a shutoff valve in front of both coolers. Run it on the track all 3 ways and check your temps and pressures.
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