Engine of barn find-steps to evaluate/save

Well shoooot... I tried the "hose in the cylinder" test. The front 5 seem to show up and down movement, BUT that last cylinder, in the back, seems to be staying in the same place. The hose, on cylinder rotation, jiggles a little, but that could be a valve moving it. How difficult is a rebuild on these? For all I know the auto trans could be trashed as well.. Body has plenty of rust to fix. At what point is it just not worth dumping money into?