The earlier engines can be drilled and tapped for an OP gauge. I've done it several times, even on A10's. I like to use 1/16" NPT for doing it because imho space is limited.
Nice looking valve arrangement but I wonder about the oil pressure pushing against the exposed part of the rubber backed washer. It seems to me it might try to move it some and find a leak path there. Does its retaining clip seat in a groove?
Another concern would be the life expectancy of the rubber seat. Why not use a metal seat like most other relief valves out there? Accurate metal to metal can seal quite well.