Here is what Vixen has. It is a 1/8" port. View attachment 1715950328
Next is the original 170 I pulled out of Vixen. It is also 1/8" NPT.
Finally is my 1977 model closed chamber prototype head. It has two 3/8 NPT ports. So you could say the people you talked to were WRONG. View attachment 1715950330
that hole around the corner is 1/8 NPT on the 77 head, if it were me I would put the stock sending unit back in that hole and hook up the auxiliary gauge via one of the 3/8 npt holes
