Holley may print that but it is mechanical nonsense....there is nothing special or different about any fuel pressure regulator that makes them "electric fuel pump only" or " do not use with mechanical pumps"....that's just hogwash.
Think about how a regulator works....it doesn't care if it is fed pressure from a electric rotary pump, , electric plunger pump, mechanical plunger pump, or a fuel tank mounted 25" above the car using gravity feed to provide the pressure.
Gauge location doesn't matter either, a gauge located anywhere on the output side, be it in the regulator, in the line with an adapter 1/2 way to the carb, or right at the carb inlet is going to all read the same pressure.
The ideal mounting point for a return regulator is after the carb....line goes from pump to carb feed line or log, from back end of feed line or log to regulator, and then to return line.
This way, if the carb needs ever drop the pump puts out, nothing is sent down the return, return is only active when there is pressure above the set regulation point, thus meaning extra fuel is present so the return opens and sends the excess out the return port.
First link is a little oversimplified at the carb inlet not showing the feed lines or log...but the second link shows exactly how it should be.