Dude! I just ran through the exact same situation with my 88 G63B.

I bought that exact same holley regulator and a harbor freight 0-10 psi gauge. Couldn't get a reading, was frustrated, truck ran like garbage, already tried re-jetting. Finally decided to listen to everyone's advice on here and put an electric pump on my truck. Turns out I wasn't getting a reading because my mechanical pump was putting out 0.5 psi at best. I have a shortbed regular cab 2wd which makes mounting a fuel pump near the tank somewhat difficult, it's real tight back there.

I used an Airtex E8016S. I know Airtex doesn't have the best rep but I had it laying around from a different build. It was something like $25 from rockauto, I looked it up for a first gen RX-7 that uses an inline pump, it puts out about 4-5 psi. Best of all, it's tiny and has a pre-filter threaded right into it. Don't forget to hook up a relay. Echlin part no. AR272 (standard five-pin relay, fifth/middle pin goes unused) works perfect.

Truck runs much better now at idle and on the primary jet. Could hardly get into the secondary range (>3000) before. I really think the mechanical pumps are garbage for anything but the stock carb (which also happens to be garbage lol).