Looking at buying a '72 340 Duster, but the tail panel is pretty trashed (not dented, mostly rust & bondo). I have a lot of experience with floor pans and patch panels, but the tail panel is a little daunting. How hard are they to replace and keep everything aligned (I'm thinking a temporary brace will be required). Thanks!

I'm getting ready to replace one in my bro-in-laws 72. There are numerous spot welds to drill out around the perimeter. YES, a brace along the top edge would be helpful.

400 or so spotwelds to drill out. I didnt brace the Demon I did since the trunk floor was still in, turned out fine

I replaced my tail panel on my '71 340 Duster.
First thing. The one from AMD is wrong. The panel will fit OK, but the trunk lock stamping is wrong. It's ok if you don't care about it being right. I got my tail panel from a friends car (wait till he finds out it's missing, ha!). It takes a lot of spot weld cutting and time. But just go at a slow pace, and take it easy. Another thing is, spot welding it back on. We used a spot welder with a long cable, hand held lead. The ones with the crab type pincers will only work to get to some of the weld spots. As far as something to hold the car in square, I didn't use anything. The trunk floor was never taken out. Everything came out square and trus. Just check it every so often.
Here's a pic of the tail panel I used.
Also, check for any rust spots on the metal your going to spot weld to, and replace it.