If the rear end hasn't been rebuilt, you might want to take a look at your bathroom floor.
There is usually going to be some floor rot that needs to be addressed (structural stuff) before you start the restoration process.
I've been through a '72 Overlander
and a '73 Sovereign
and they both had significant rear floor/frame rot/rust that had to be repaired before proceeding.
Don't be afraid to remove the belly pan to have a look. (If you have odor that you'd like to be rid of, that's a good place to start.)
The finished product
is worth it!