I have the big GZ pump on my 505 C.I. low deck bracket motor, it barely pulls 5 inches revved up in nuetral work down
I have bought but not tried a Moroso three and another four vane pump, hopefully one of them will pull more luck
All three where used so maybe I need to return the GZ to them to check it out, I called them and they say it should pull more shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)