I've replaced/installedf sending units on my '66 and '68 with the tank in situ and on the ground. IFF your tank is CLEAN, relatively NEW, and/or otherwise RUST AND DIRT FREE, then leaving the tank in place probably will be easiest. The original lock ring comes loose with moderate torque, provided you use liberal amounts of a good penetrant oil gor the purpose. Be SURE everything near the sending unit is CLEAN CLEAN CLEAN before removal. I used a large flat tip screwdriver and large Channellocks the first time and it went well enough. Since the first sending unit I tried proved apparently worthless at first, I had opportunity to replace it a few weeks later with a second one from a more reputable vendor, and had obtained the prescribed lock ring wrench in the interim. THAT expedites removal, and more important, PROPER REPLACEMENT well, so spend the $10-15 for it, and sleep better.
That tank is best EMPTY when doing this little job. Make it so.
I loosened the straps by turning the nuts on the J-bolts down to near removal. That sufficed for maneuvering room and also replacing the fill tube grommet. THAT job is FAR easier with the tank on the ground though it can be, and was accomplished the first time with the tank hanging above me the first time. Its your call, though the precise model of your ride might be a decisive factor here.
Mind you, our '66 came with a very NEW tank in March, 2016, though the doltish boy who assembled the car from its components as he found them in his deceased daddy's garage had re-used the old components with it. I understood this mystery better when we first got our '68 Newport, early October 2021. It had the original tank alright, which some vaquero had tried to repair on the severely corroded top side with some heavy plastic coating, likely made for the purpose, some decades prior. The rats managed to chew through some of that, invade the tank, and perish there. Suffice to say, I went through quite a few fuel filters the first few weeks of driving that car. That circumstance was what compelled me to remove the old tank and replace it with the still shiny, CLEAN tank from our wrecked '66 Newport. I then came to see how busy the Family Guardian Angels had been, saving us from death by incineration via an extremely compromised gas tank. Since the plastic repair coating stuck it to the trunk, enough of that peeled loose upon removing the rotten tank to allow me to see the rat carcasses in that execrable tank.
I placed this relic by the street the evening I removed it, and ECCE!, it was gone by dawn following.
Suffice to say, I took the liberty of replacing the filler tube grommet, sending unit gasket, and the factory original sending unit lock-ring from that rotten old '68 tank. THAT proved to be the ONLY portion of that tank worth keeping, while the old lockring from '66, while heavier than the thin sheetmetal comprising the sending units I had purchased, still appeared to be after-market compared to the clear original off that '68.
All told, I consider dropping the gas tank more essential to the purpose of replacing filler tube grommets, and/or filler tubes. Turned out, the vaquero/flyboy mechanic that mis-maintained that car while it remained in the USAF had used some filler tube NOT for a 1968 Mopar C-body vehicle, and I availed myself of the 1966 tube after it soon became clear what I had reinstalled to the otherwise good job with the new tank was as WORTHLESS as I suspected it would be. With the '66 filler tube, a good grommet, and the fuel sending untit all in order, we've enjoyed use of a good gas tank sans major trouble for over a year.
I replaced the filler tube grommet the last time with the tank still firmly attached, as I was in a gas station parking lot. You DON'T have to remove the tank on slab sided C-bodies to service either end.