You need a PS pressure tester plus a a Borgeson shim kit, or send the pump out to be set up. I don't see any way you can guess at it. If you have a DSE rack, it can handle 1400 psi, but Mustang racks use 950-975 & I think 1000 or 1100 is max.
A local mechanic may have the needed gage kit & be able to set it for you.
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