I have been following the sagging rear end threads. Certainly a problem as things age.
I had a 66 Fury on which I had the rear springs re-arched and also air shocks installed. Pretty much still sagged. I had a 66 Newport in which I installed rear air shocks which help, but still not great.
On the other hand, My 69 LeBaron Coupe sits with its ass high in the air and is very firm at the back. It is almost too high, but I am not going to complain. It has a factory tow package according to the Broadcast sheet.
Fred Joslin