I need to change the filler neck on my Duster as someone who owned it previously completely ruined the lip where the gas cap twists on. As a result, the gas cap does not tighten so there is no seal to the neck. I picked up another filler neck and was wondering if I can change it as-is? If not, will I have to take the tank completely out or could I get away with just loosening the straps and partially lowering it?

Try it, that is how we learn, correct
I built a 1971 Duster from the ground up years ago and had to replace the original gas tank later when the car slipped off the floor jack onto one of the jack stands under the rear end ruining that tank
What I remember now is the hard part is getting the filler neck to slide back into the rubber grommet in the tank

Grease it good.

I have done a couple of A bodies recently and my thought would be that you'd be money ahead to just save yourself the headache and drop the tank, the filler neck goes pretty deep into the grommet and in my mind trying to snake it out and back in without dropping the tank would be a pretty good tail whipping

Leave the tank in the car. The neck has to go through the quarter panel on the Duster. It will go in from the outside into the trunk then into the tank. There is a flange that the retaining screws go into the quarter panel. The gas cap covers all of this when you put it in. Just use some penetrant on the grommet. Don't forget once the neck is partially through to in the truck seal. But no taking the tank out will not work on a Dart Sport/ Duster or Demon.
Thank you, Ken

My 1971 Duster filler neck had three screws that held it into the 1/4 panel, it did not have to be slid in from the outside of the trunk
I don't remember how the filler neck installed in my Wife, now my Grandsons, 1973 340 Space Duster
My 1969 Dodge Dart GTS installs the same way my 1971 Duster did

I'm with Cab...all the Abodies I can recall the filler neck flange is located on the inside of the quarter panel I just completed a gas tank replacement on a 67, 72, 73, and a 74 and at least on those the flange was inside the quarter panel, on all four I removed and cleaned the filler neck from the bottom of the car , as a side note it's the same on both tanks on the Dually I'm restoring.

Yep filler tube goes on the inside of the quarter. Have to drop the tank. Use dish soap on the seal to reinstall..

I'd probably get a new seal as well.

I've got a 71 in the garage that I'm fighting with now.

FWIW my car came with a filler that mounts to the outside of the quarter panel. It went right in with the tank strapped in, but the gas cap stuck out off the side of the car. I believe those are for later model Dusters or something but they won't work with the 70-71 quarters at least The gas caps are different too.

I bought the right filler and cap for my car, but can't get the tube to go into the tank. The FSM is no help with installing it either. The only specific instruction is to rotate it 180 once you get it in the tank (or before you pull it out). No wonder they redesigned it, because it sure seems impossible to get the filler in the tank to me. If anybody knows the secret, I'm all ears.

I've tried loosening the tank a little to try to change the angle the tube meets the tank but had no joy. I guess my only option is to drop the tank and try to get it in. I'm not even sure the thing will reach all the way through the trunk grommet down to the tank, if the tank is on the garage floor, but here I go to find out.

I had to modify my 1971 filler tube to clear the full wheelwells I had added with the chassis work, I ended up having to replace the original tank several years later with a new one from Spectre after damaging the original one I ordered a new filler grommet also.
That was bunch of years ago, it was a hassle but I did get it by using some wheel bearing grease on the tube and grommet and determination to get it done.
I'm thinking I had the straps loose until the tube would fit properly, I didn't look in the FSM so maybe I got lucky on making it fit in their with the twisting and shoving
You can do this
If the factory guys and gals can do it you can to
Don't stop until your happy

Yeah I've been fighting with it off and on for a couple of days. Today I thought I was getting somewhere with the tank loosened up, but it turns out all I'm doing is shoving the tank downward because it's loose.

The later design that goes through the outside of the quarter is the way to do it, if you don't mind the stupid looking fitment of the cap.

FWIW I've lubed it with motor oil, then read somewhere to not do that because it degrades the rubber and to use ATF instead. OK, that doesn't work either. Today I gave it a couple of healthy shots of silicone spray lube, still no improvement.

Heading back out to fight it some more now. Man I love working on cars /s

At least it's a new tank, so there was no siphoning involved and I'm not sloshing gas in my eyes etc.

ETA - if you don't make sure you've got it lined up with the grommet, you'll bend the tongue that sticks out of the end of the filler when you twist and turn the filler. DAMHIK.

ETA2 - if you bend the tongue you probably cut the grommet and will need another one

I find the easiest way to do this is drop the tank out, install the filler tube, then from below, align the tube and grommet, and wiggle the tank up into place

Jeez I can't believe I had so much trouble, but I think it all boils down to standing in the wrong place. I was trying to work it from the side of the car, when I needed to work it from behind.

Remove the floor boot.
Grease the tank grommet generously.
Put the floor boot on the filler tube. Be sure you have it right side up.
Stand behind the car about at the tail light.
Holding the filler down low inside the trunk, put the tank end of it into the grommet, and then swing the tank end around until it's aligned with the tank and the hole in the quarter. The cap end will be higher than the body hole.
Slide the filler right in.
Put the screws in the floor boot. I found it was easiest to start with the screw that will be right there in front of you, standing where I described.

Done and done. Well almost, I still have to drill mounting holes in the 1/4 and find some screws.

I was surprised that it doesn't seem like there's a lot of tube in the tank, but I'm not messing with it again. I'll try to remember to not stuff it full of gas.