This gives you a better idea of the OPRV problem:
The drilling at the top is from the oil pump (blanked off by a screw on the bottom). The hole to the left is the drain to the case. The early engines dumped it into the sump. Later the drilling goes to the drilling for the pickup tube with a ball in the pickup tube to prevent the oil from going back into the sump.
Notice the 2-1/2 - 3 threads between the base of the hole and the drain hole. Lots of potential for leakage there. When installing the OPRV you should clay the bottom of the hole to see how far the OPRV sits to the end of the threads and adjust the thickness of the sealing washer. Or you can run the OPRV all the way in an use feeler gauges to check the minimum thickness of the sealing washer.
A non-hardening sealer such as Hylomar can be used. It has to be put in the case threads between the drain hole and the bottom. You have to experiment with how much to use so you do not have a wad sitting in front of the OPRV. If it gets pushed into the piston it will probably stick it.
With a different OPRV the drain hole can be blocked and the relief oil joined with the return line or a hole can be made in the lump to the left of the OPRV hole (cross drilling for the return to pump) and route the oil that way.
I have the filters and mounts available. Unlikely will make more of the Norton mounts though.
The '71/'72 OIFs had the oil feed pipe out the side of the backbone which puts it in the way of the Charles filter. Later frames put the outlet tube in the sump plate.