Our house had a 1-1/4" drop in the middle of it--which sounds like a lot less than yours (our house is only 22 years old--but even this one has sagged more than many its age).
Our problem was less to do with the flooring than the foundation. (I should state at this time that I'm NOT a structural engineer--and that I did hire someone to jack up the foundation in a couple spots). And having a "few too many" would help--but only for a while!
In my home (which I've had over 8 yrs), it was bonehead building....shortcuts taken by the builders. And I guess they bought off the city inspectors to "look the other way" as the problems are as obvious as the nose on your face. Whatever; I'm sure this happens all the time--and a newer home is no guarantee that there won't be something wrong.
In your case, there is probably a good reason for the settling. For example (since it's 80 yrs old), is there perhaps one-too-many layers of roofing up on top. Or, one of a myriad other things...moisture problems, settling pier(s), or ????
They ended up jacking up the house almost an inch in the middle. It didn't take care of all the sagging floors--but it did help. I guess I could come along later and do some floor leveling---but I'm thinking about just calling it 'character' at this point.
Anyway I guess, depending on what you find out, the nice wide planking may be fine depending on what's down below it.
Please keep us posted. I am most interested in this topic--as I'm sure are lots of others as well.
Best of luck,
-k2 in Colorado