TOPIC: How do I order the right fuel tank sender?

Fuel gauge is on the dash, tank is under the floor. Old sender is corroded and gives low tank level readings.

Seems like a lot of the sites selling fuel senders don't specify hole diameter, bolt pattern, and some don't list arm length or ohm rating.

And speaking of ohm rating, mine is pretty well cooked and doesn't cooperate when I try to get a reading.

If it's an American factory install (1979) can I assume it's standard? (beyond arm and ohm)

Just about all sending units are calibrated to 35-240 Ohms but getting the reading to register the true level of the tank could be a problem. If you know the capacity of the tank, fill it to 1/4 capacity and see where the needle registers. That's the mark you need to know. Anything above that is clear sailing. It may not be THE answer you're looking for but it beats running out of gas.

However, the real issue is hole alignment. The best way to approach the problem is to measure what you have and ask the seller to measure his. Something like this might do:

Your gauge might say in tiny numbers what the ohm range is. If they don't match exactly they will close and the indicator will only be off by a little bit.
They are pretty universal now, the arm lengths are adjusted to your tank. My hole diameter was the same on my 1962 metal tank as the one they sent me, maybe yours will work too.