Return line fuel restriction?

^^OK that there is such a short section that "I bet" if you immediately bushed that up to 3/8 you'll be fine. It would not hurt to trim it down to minimal length inside and outside the tank. Also keep in mind that tube is smaller than hose, I'm sure you know that!!

You can easily measure restriction, just put a tap in right at/ after the regulator return fitting

As a former HVAC mech, I've always liked 1/4" flare Schrader valve fittings for test ports. They are of course very common in HVAC and there are lots of configurations plus "an old set of hoses" are fairly easy to drum up. If you use regular hex head flare caps instead of the "finger caps" they are very secure