Can I install a new fuel sender unit on a 72 duster without removing the tank?

yes totally agree on the lock ring tool. I just replaced my tank and even the new AMD supplied ring of questionable thickness was hard as heck to get turned all the way into place. had to hammer the tangs where the tool can put even stress on all 3-4 and just turn them in with its long handle. Probably use it 4 times in 20 years but worth it.
MF293 - 1962-76 Mopar Lockng Ring Install Tool