My home has a gas wall furnace. The wall just above the furnace gets very very warm. I realize there is a pipe behind the wall that vents the heat and will get hot but, how hot is too hot? It's an old home and the furnace is more than likely the original (40 yrs? )

Absolutely cut open the wall and call your town building inspector to look at it.
Codes are much more strict about venting than they were 40 years ago.
It might not be the heat of the wall per se that is dangerous, but other things might be.

Make sure you vent with b-vent pipe and you have 1in. Clearance from combustible material and you should be fine