My friend has a factory hp mech pump with a return line. Does this return cary gas back to the tank or is it a early smog deal that was for vapor only? We're trying to figure out if it could be blocked or needs to be ran back to the tank.

I would just run it back to the tank and don't plug the return fitting on the fuel pump you might make around 14 lbs of fuel pressure and blow the diaphram or float trouble.