Does the 57 have the hydraulic cylinder assist like an early Corvette or a servo on the gear? Generally, I think the cylinder style take less pressure.
I have a GM Power Steering and Pump service manual from 1975. It has the following information:
Chevrolet (except Vega, Monza and Corvette), Buick (except Skyhawk), Oldsmobile (except Starfire) and Pontiac (except Astre): 1350-1450psi
Cadillac 1425-1475psi
Vega, Monza, Astre, Starfire and Skyhawk 750-850psi
10-20 Series Vans, Corvette 900-1000psi
10-20-30 C & K Series Trucks 1200-1300psi