Hey guys, I have a fuel lab pressure regulator. I am wondering if anybody has the specific fitting that threads into the regulator that fits for the stock fuel return line. Would it be a NPT thread?? Please and thank you

Which model do you have? Part number?
It would depend which model you have but usually the inlet/outlet threads is ORB (The same as AN threads but w/ o-ring) which means you would probably need a 6/8/10 ORB male to 3/8 barb fitting if reusing the stock return line. Most Likely 6 ORB to 3/8 barb.