Todd,
You can also install shim washers in the pump flow valve.
Bill's Hot Rod in Covina, CA makes a shim kit to reduce the GM pump pressure. The Mustang II rack operates at about 900 PSI, the GM pump puts out about 1300 PSI. You would need 3 to 4 shims for it to operate correctly.
FRed

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine