Hello Everyone

This is my first post to this group and I really hope that someone can
help me out.

Our house is a bungalow built in 1962. In 1964 an addition was added
to the end of the house. Evidently the ground wasn't compacted and
over the years it has sunk. The sinking is most noticeable in the
dining room part of the living/dining room, where the drop is about 1
3/4 inches over 6 feet going West and about the same over 16 feet going
North.

It isn't feasible to try jacking up the house so we seem to be left
with leveling it and this is where my questions start.

I have read about gypsum concrete as being cheaper and easier to work
with than leveling compound. Can we use gypsum concrete for a leveling
compound on wood?
Is it a good idea to build up the deeper areas with OSB, Tar Paper or
something else?
Should we do the leveling in zones?
Should we try to feather the leveling compound where it is thinnest or
is it better to cut out some of the sinking subfloor so that the
thinnest leveling compound is a minimum of 1/4 thickness?

Having written all of thes questions it occurs to me that they could
all be completely stupid. If they are would someone please suggest the
best way to go about solving this problem?

Many thanks

David