Oh boy, I love it when one of my challenges results in vigorous information/idea sharing! It's a good sign I'm dealing with a non-trivial issue and that I'm not an idiot for not already knowing how to resolve it. So please keep the information/ideas coming.
So the plot thickens...
I dropped the gas tank and it's clear now to see why I went with the rather hacky fuel return line into the tank vent configuration. This gas tank is very old school and was never meant to support EFI. It has the filler pipe welded on the side, a hole for a SAE 5 screw fuel level sending unit and a vent welded in place. That's it. I had to modify the sending unit to add on a fuel line to get the fuel out of the tank.
The tank hole for the sending unit is 1 1/2 inch and the Stewart Warner sending unit is located in that hole filling all of it. Back in the days when building this car, I somehow found a way to drill a hole through the sending unit to pass the gas line through. I was hoping there would be room for a second hole for the return line. No luck, it just won't fit even if siamesed with existing line.
I've done some Internet searching and haven't been able to find a SAE 5 screw sending unit that also has fuel out and return lines. I've found OEM sending units that have fuel lines incorporated into them but they are not SAE 5 screw mounts and I suspect they wouldn't fit through the 1 1/2" hole. I found a SAE 5 screw plate with 2 threaded holes for fuel line out and return but I suspect it would be very complicated (likely not possible) to graft on a sending unit and still have the works fit through the 1 1/2" hole. Did I miss finding the "magic" part? Do you guys know of a combination sending unit and fuel line in/out part that has a SAE 5 screw mount?
I also found a couple of fuel return line port kits that require drilling a hole in the tank. One requires you can get a large sized wrench to it to tighten the sealing nut and with a 1 1/2" hole I don't see that as working. It has an option for running the return line to tank bottom but this also requires getting a good sized wrench into the tank. So I've ruled out this option.
The other attaches like a Riv-nut so all wrenching is external to the tank. But given that, it doesn't support attachment of line inside of tank so return will be to top of tank only. I think this fitting might fit between the tank top and under the trunk floor with a special 90 degree AN fitting but locating it in just the right spot would be key.
Following the theme of keeping it simple, I've come up with another option. That is adding a fuel return port into the "S" bend filler pipe shown in picture above (shiny section is where overflowing gas dissolved the paint coating). Obviously, this doesn't get a return line to tank bottom but neither does the other viable return port option. It could be positioned so the return tube runs down the filler pipe and releases fuel below the tank top level. It's simpler in that it doesn't require a new hole in the tank with the possibility of getting shavings in the tank or having the new fitting leak. There's more space to work with out in the fender area than between tank and trunk floor.
Is there a reason that this is a bad idea? The return fuel would be taking the same path to the tank as when new fuel is added. The gold plated option is to retrofit the tank with an in-tank fuel pump/return system but I'm into a pretty major project at that point. I really don't like drilling/cutting into a fuel tank that's had fuel in it if I really don't have to. I don't have the same reservations for drilling and welding on a section of filler pipe though. I do understand that in addition the tank venting needs to be fixed or I'll still have fuel coming up to the cap.
Is the simpler option the better one here or am I missing something?