Is lead an acceptable filler to reform an area of sheet metal that is impossible to fully access? I know the factory used it to fill and smooth seams where some of the body panels met. I've never used it before but it seems like it would be less prone to cracking and rust than plastic. I realize there's no structural strength to it but maybe can be used to reform contours in difficult to get to areas that need a lot of work. I dont really want to lay on that much plastic.

I would be inclined to use the tools out there that weld a stud to the sheet metal and pull out deep dents or low spots with a slide hammer.
Lead probably would work better than fillers like bondo but be very sure you are aware of the dangers of lead fumes. OSHA has regulations covering lead exposure which I would read. At one foundry I was familiar with, the guys exposed to lead fumes would work for 15 minutes and then get away from that area I believe for and hour. As far as suitability for the intended purpose I think the lead will work certainly better than Bondo. Just be sure to be careful of your health while working with it.

I removed the lead seam in my driver quarter when removing it. recommended heating with a torch and using a wire brush to push the lead off of the seam when melted. Don't use a grinder to remove you'll put all the lead dust in the air.

Eastwood sells a non leaded formula to use in place of lead.

Lots of great Youtube sources on this topic. I was curious so I looked. Search "leading a car"

Thanks. I'll check it out.

Looks like that right rear took a pretty hard hit from the rear. Pushed in the tail panel a bit too. If I remove the cocktail shaker brace I think I can get up in there and bump some of it out. Not going to be easy though.

Back in the early 80's I used a product called alumalead. Came in the form of a powder mixed with a hardener.
Don't even know if it's around any more. Used to fill seams. Had to sand it with different grades of sand paper.
Had to apply it as smooth as you could and work with it to level it out, took some time, but the end result was pretty
good.