Sorry, I should have been more clear about my setup. I'm planning to run a gen 2 coyote. I have 3/8" stainless steel hard supply and return lines. The braided flex lines (Aeroquip PTFE -6 AN) connect the fuel pump hanger, filter and regulator to the stainless steel hard lines. What I would dead-head would be the braided line coming off the pressure regulator that would eventually run to the engine, so there would still be a supply-return loop (pump-filter-regulator-back to tank) that fuel could cycle through.

I've seen your link to the Fragola pressure test kit before - thank you for that! I purchased one based on your recommendation, and I've been testing each individual flex line after I assemble it. But it occurs to me that these are just one small piece of the puzzle, and testing this way still doesn't test the connections from flex lines to fuel pump hanger, filter, hard lines, etc. What I'm hoping to avoid is a small leak at the hanger, for example, that would require dropping the tank later to address but I could've easily dealt with now when the sheetmetal is not in yet.