That may be too small. I had an SX EFI pump and regulator that would adjust to 5 psi. The key was i needed a #10An feed and a #8 return line. I tried to cheat with smaller lines and could not get it to go below 10 psi.
Please dont be offended, as i dont know you? You have the lines going to the correct ports on the regulator? As i said the smaller line may be causing you issues. I dont think that small 5/16 line is going to supply the volume of fuel needed for your application. You may end up running a new 3/8 or #6 An line from the rear to the front. I ordered a fuel tank pickup with a 3/8 pickup and then plumbed that to the pump, then ran #6an to the carb. no issues.