just in case anyone is interested, this is how I took care of this issue. I screwed and glued 2x4s all the way around the inner perimeter, and doubled them up on the opposite side.
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Than I went over the existing frame and the bridge I built with a 1x10 on one side, and a 1x12 on the other side. (The side that I doubled the 2x4's on) I also caulked all the seams.
Than I spread on a construction adhesive on the frame and stapled the liner in place. Then went over all the seams and staples with tyvek tape, and caulked the surrounding seams. I would of preferred to use caulk on the seams and staples of the liner but since I still will need to shim the frame when I put it in, it doesn't make sense to have shims sitting on caulk. Plus caulking makes a big mess when you can't get the seams to lay perfectly flat and tight.
Basically I just did what xamdam told me to do and it came out pretty sweet. Thanks again man.