Mounting fuel pressure gauge

I don't know what car you're working on so I'll wing it with a response. You can reach the underside of the cowl by taking off the fresh air box under the dash. It isn't that hard. If I'm doing what you're trying to accomplish I would drill and tap a plate with the bolt spacing you need. Then use a temporary type glue to attach it to a screwdriver. Slide it into the cowl through the fresh air opening and have someone watch through the cowl and guide you to where it needs to be. Then they can screw the gage into place. Twist the screwdriver loose and you're done. I don't think you are getting a plate and hand into the cowl.
Good luck. Sounds like a job that will require adult language.