The tubes are steel and they will rust through with a pin hole size so small it's tough to find.
The original pickup hole needs to be plugged. It is allowing moisture into the tank every time you wash the car.
Most likely the newer pickup has holed through also.
If you are going to let the car just lay there for long periods of time, best to do it with a FULL tank of gas.
Those rust holes tend to appear at the level the gas was at for extended periods.
Wilkinson has new pick ups/senders for $169.95. He is the sole importer of that part and if you want to pay more to another vendor like Hall who sells it for $230, and gets it from Wilkinson, it's your choice.
I JUST went through this with my car. It is the pick-up TUBE IN THE TANK.
http://www.panterapartsusa.com...l.cgi?prod_id=09003A
Incidentally with these corrosion issues apparent in your tank, you should slosh it, then tefflon coat the inside of the tank.
The tanks rust out too. A new steel one is $995 and a stainless one is $1100, but you need to pull the engine out to change it.
I sloshed and tefloned mine in 1986. I was just in there with the procotology scope I borrowed from the OR and the teflon is still in place with no sign of rust anywhere. No polups either!