How should I finish a basement with a pitched floor?
I would stop and make your floor level before you proceed.

Home Depot/Lowes/etc stores in my area sell 2'x2' subfloor 'tiles' that is basically tongue &amp; groove OSB with dimpled foundation membrane on the bottom side.  However, you can do this yourself using the foundation membrane and sheets of plywood or OSB.  The flooring looks like this: 



As I see it, you have two choices:


Leave your subfloor as is, but install sleepers and plywood or OSB on top (will reduce ceiling height a bit).
Rip out the subfloor, and install something like the dimpled foundation membrane, then the sleepers and the plywood or OSB.  This is better, in my opinion, because it would allow any water to run downhill instead of being trapped through capillary action.