In case you aren't familiar with aftermarket sending units see this thread for advice:
Fuel tank sending unit for 1966 Dart?
You will have to do some tinkering to make it work right. At least you have the new tank out of the car. When you get the aftermarket sending unit make sure the pickup tube reaches all the way to the bottom of the tank. My aftermarket sender wasn't anywhere near the bottom of the tank I had to bend it 2-3" down. You should be able to test fit it and check the pickup tube by looking through the filler neck hole.
Don't throw away the old sending unit. It looks rough but can be restored. You could sell it on here easily.
