well any fluids in the passenger compartment are not legal for NHRA and may not be legal for SCCA either (not sure how strict tech is). The legal way is either an electric gauge (doesn't seem much more accurate than factory, just has numbers on it) or an isolater (real pain in the ass and expensive). So it looks like your only option is mounting it on the hood. Don't forget a tach while your at it

Why do you need an oil pressure gauge for that thing anyway?