Andrew,
A Fluidyne DB-30618 is the oil cooler I ended up with in the plane.
When a pressure gauge was installed in the line between the front cover outlet and the RX7 oil cooler inlet, the pressure rose with increasing RPM to 150 psi at 2800 RPM. Increasing RPM above 2800 resulted in a steady pressure at 150 psi. Since the oil
pump is a positive displacement pump, additional oil flow above 2800 RPM passes through the relief valve into the sump. At 5600 RPM half of the oil pumped by the oil pump never flows through the oil cooler.
With the Fluidyne oil cooler, the pressure at its inlet never reaches 150 psi so all of the oil passes through it.
An archive search for 25 Feb 2014 should find a description of various oil cooler configurations that I tested.
Steve Boese