On a typical Melling 60 psi pump, a 1/8" hardened washer under the spring will get you right about 70 psi.
Be sure that whatever you use to shim the spring is hardened steel.
Also note that there are, or were at one time, 40 psi pumps available. In the old days these pumps were physically different, had smaller pickup tubes and possibly different thickness gears (not sure on that). In any case, shimming one of those or replacing the spring wouldn't work well, better to get a new 60 psi pump if that's what you have.
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'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 350/4-speed Black on Black, mix & match.
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