Today I started redoing my old closet. The ceiling was trashed from a
long along roof leek (since fixed). Anyway, I decided to sheet rock 2
of the walls, leaving the other wall (which is lathe) alone. While
taking some measurements, I discovered a "problem".

The closet is rectangular shaped, with the door offset to the right.
When you enter the closet the chimney is on the left. Behind it is the
HVAC duct work for the 2nd floor of the house. About 1' from the
ceiling is an oval duct that fees the bathroom.

What I want to do is to put up a wall on the side of the closet where
the chimney is. The problem is that if I frame it so the drywall sits
flat against the chimney the ductwork is in the way on the opposing
wall. In order to clear it I'd have to move the wall 3" over from the
chimney so it will be flat. if I sit it against the chimney (using
some firring strips) then I'd have to notch one part of the drywall.

Would notching the drywall be acceptable? I know it's only a closet
but I've never undertaken this type of work and want to do it right
and a closet is a good place to get some practice in. If that is
acceptable, how should I seal the area were the drywall clears the
vent? The venting is that flexible stuff.

I don't have a working camera, if need be I could draw something up to
illustrate what I'm trying to do.