I may be over thinking this but blocking this off and using the 3/8 NPT is a large restriction. I have an alum block that the filter mount was milled off and 3/8 NPT was tapped in. The ID on that fitting is only about 55% the area of the 1/2" passage it feeds and is supplied from.. I drilled and tapped for 1/2" so the inside of the AN fitting is very close to 1/2", around .490. If I needed a blockoff plate I might try to tap into it with 1/2"-90AN fitting for the larger hose and less restriction. If they will clear.
I use the upper hole to feed in to my block from my remote filter. I drilled & tapped the hole to 1/2npt and use an AN-12 fitting. I also drilled the fitting out with a 1/2" drill. I have a belt driven oil pump, so I blocked off the feed to the original filter mount, where the oil pump used to mount. Then, for the return hole where the filter used to mount, I just plugged it with a short 3/4-16 bolt.
