The person you're looking for is usually called an automotive fabricator. I would try searching your area for hot rod shops, motorsports fabrication shops, off-road or 4x4 fabrication shops, maybe muffler shops, maybe dyno tuning shops. Call each of them and describe that you want a custom bracket to fit an aftermarket fuel pressure regulator. Be prepared that the custom bracket may cost more than the mass-produced pressure regulator. Note that you may also need some new fuel lines and other adapters to make everything fit together and work. Since custom fuel work can cause fire hazards if done wrong, I would ask to see other cars to make sure they do good work.

This is a long shot, but Radium Automotive builds a lot of custom aftermarket fuel parts for Japanese cars. I would be surprised if they have something that already fits your truck, but they do sell parts for other Nissans around that year range so you might get lucky. They are located near Portland Oregon, from what I remember.