Either something has gone bad in the manufacture of these over the years or your sample of one was a weak contender.
I have a flow graph from Airtex dated 4/4/92 (the scan is too big to attach). It is the 3270 at 13.5 volts. The results they got from their samples averaged from the production line is quite different. It's not very readable so here are the numbers:
psi -- gph
0 60
12 55
32 50
46 45
55 40
61 35
65 30
70 25
72 20
76 15
81 5
85 0
Seems about enough for 420 hp (assuming 100% duty cycle).
those 13.5 numbers are close to my 15 volt numbers------but they fall quite short of a stock TR pump......RC