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.