You cannot reliably get the fuel sender out of a tank with it in a Pantera without bending the float arm, with a high risk of breaking the assembly. You should first remove the left side rear window and trim (it only pushes in). Once that's out and working thru the now-open window space, you can direct the assembly out and back in, clocking it to miss the internal tank anti-slosh baffle. Both Wilkinson and Larry Stock/PPC have brand new late model senders in stock..The early type is no longer available. They are not expensive.
There are two different senders- early and late style with a fuel feed built-into the late one. There are also two floats- brass and plastic foam. Both have given trouble. The soldered brass, like similar ones in carburetors, can develop a leak and let gas inside which makes it heavier. It can be heard by shaking the float. There is no practical fix. The plastic foam is coated and if the coating is damaged, the foam will soak up gas and also get heavier. Again, no reliable fix.
But a float from a sender for any FIAT or Alfa interchanges with a Pantera, since they all use the same sender with a different float-arm for different cars. A foreign junkyard is your best source for just the float. Very SLIGHT bending of the middle of the long swinging arm will radically change the gauge calibration. Rough handling or mauling the thing getting it out without removing the glass will usually break the arm at the 90 degree bend coming out of the little box. The sender mechanism is just a simple potentiometer and the tiny internal wires sometimes break after 50 years. Such a sender is junk.
Note- the stock fuel lines from the tank or late sender are less than 5/16" ID, so they cannot flow enough fuel by fuel pump suction to support more than about 450 BHP continuous. All the above was the subject of several past illustrated articles in the nearly 600 archived POCA Newsletters going back to 1973. Also note- I no longer fabricate custom senders with much larger ID fuel lines for higher horse engines (Silver State ORR, Bonneville etc).