Just be VERY cautious, on a saxo that was fitted with a combined reg/gauge the pressure bounced all over the place from 1 to 6 bar, couldn't map it obviously, so it could be a very costly mistake.
If you are just after a gauge, then there are decent ones starting at a low price, I use a £15 one in my pressure tester, this has been checked on a master gauge and was as far as eye could see bang on at all pressures 0-10 bar.
It has a 1/8NPT thread so you could just get a 1/8 boss welded to your rail, put gauge on job done for not much money, you keep the excellent w/m regulator that way as well, so no hassles, if you want it remote, ie in your car then simply buy a remote kit with gauge for around £50.
tabetha