Not sure what might be wrong with the tank selector hardware, so someone else will know more than I do there.
What I was referring to was something I got into years ago when I changed fuel filters and in the process, a piece of neoprene gas line about half the size of a BB broke loose. When I was driving, the fuel pressure forced the rubber through the gas line to right below the seat, obstructing the fuel flow. The thing would literally run out of gas and would only idle until the bowl filled up again. Some gas got through, but not enough to carry the car at cruise speed.
Turns out my issue was a combo of things, first the regulator was not functioning, then when pressure was correct the volume was too low. There was a kink in the fuel line from the tank where it exited the pump assembly, then the tank selector was malfunctioning so gauge was not accurate. Solved all those to find the new rail pump was not pumping enough fuel. Replaced that with an Edelbrock pump. Mine needs about 35-38 GPH and 7# at the carb which is a new Holley 600cfm.
So, check fuel pressure, and volume at each component. Holley also suggested a clogged fuel screen in the inlet fittings at the floats but that was not an issue.
Depending on the condition of the tank pump assemblies, you can just replace the pump itself without replacing the assembly. Front assembly is still available, I think I got the last rear pump assembly in the world. Took 6 months to find it.