any chance you can post a pic?

I second on the pic, you must have a 3 way fitting but then I would wonder how correct would the pressure reading be at your test gauge. You still have to flow oil thru the output from the oil filter side regardless so the engine still gets lubrication internally. Also what happens if the oil filter goes into bypass ?.

Cant get pic today, 1/2 pipe female tee to filter with nipple, 1/2" male to # 12 AN male for oil line, 1/2" to 1/4" bushing for gauge. Had a filter internally collapse there was a large difference between the block mounted sender and the mech gauge. Likewise, when senders go bad they give false readings before they quit. Gauge is always there for added comfort zone.

Pretty sure it's 1/2", been a long time, might be 5/8

I see you mounted a small analog gauge. I thought you were talking about a hand held mechanical oil pressure gauge and the outlet port was were you did the test at. .