S13 SR20 Greddy Oil Filter Relocation, gauge help.

[hks240sr20]

Ok so i've got a Greddy oil filter relocation kit and I purchased a Marshall pro-street oil pressure gauge which came with a oil pressure sending unit that has a 1/8 - 27 NPT fitting on it. When I goto put it in my easily accessible location it just barley doesn't want to thread in. So I take out the larger plug that is next to it which I can only assume that it is a 1/4 NPT, so I get an adapter to go from 1/4 to 1/8 in, and I put the gauge in but it is leaking oil around the 1/4 in NPT portion of it which is in the greddy relocation kit. Now with that said my question is, is that larger size plug really 1/4npt? Also is there an easier way of getting my oil pressure sending unit hooked in?? I was basically hoping it would just screw right in but it just barely seems to big, am I doing something wrong? Any help would be great, Thanks