Just about every older MPFI (up to about '96 depending on the application) GMs were 43.5 PSI for base fuel pressure. IIRC there were a couple early exceptions.

Since you will be tuning the ECM, this shouldn't really be an issue, especially with how little the flow rate will be effected by 3 to 4 PSIG.

in an MPFI set-up you really need to use a vacuum reference to ensure the delta pressure differential remains constant throughout the load range.