Whats inside the gas tank? At evap barb.

I would. There are some sender units made (aftermarket) with a second fitting. You can measure the backpressure if you can rig a tee fitting and gauge You (I'm sure) know how I'm "redneck" On my 67 I took an awl and punched (not drilled) a hole into the drained tank, then filled the tank with CO2 from my welder bottle. Then just silver brazed a scrap of copper in there. Use your own judgement.

I look forward to anytime I can use an awl on my car. :)

Thank you again.
Guess ill just lay under there and do
some thinking.

Regarding sending units... something like this? Is the second nipple (1/4) high enough for a return line? Seems low... But perhaps that doesnt matter.
Right Stuff Detailing YSU6901 Right Stuff Detailing Fuel Tank Sending Units | Summit Racing