It looks like Holley still offer one that'll do 3psi, see
here. I remember the Purolator dial-adjustable units, too (I also remember having problems with them...they tended to stick closed and you'd have to open the hood, turn the dial to the maximum pressure setting, crank the engine, then dial it back to where you wanted it) but they seem to have disappeared. Probably wise to avoid the Mr. Gasket dial-adjustable regulator — I find an uncomfortably high number of "leaked and showered my engine with gasoline" reports about that (probably Chinese) item.
The Holley regulator has 3/8" pipe thread inlet and outlet fitings, so you'd just install the appropriate NPT-to-hose-barb fittings for your fuel line size and off you go.
But, I found at least one report of problems with the Holley regulator, and there seems to have been a shift in recommendations, away from 3psi + regulator and towards 4psi w/o regulator. You may want to take a look at
this page.
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