The th325 is the weaker OD version of the th425. You will not be able to run a c5 diff off of it. The TH325/TH425 runs backwards at the output shaft of what a normal transmission runs. Also even if you was to flip the diff to get correct rotation the range of motion of the axles from suspension would smash into the oil pan.
I have done this with a LS3/TH425, there are a couple differentials that you can use to get it to work but it still has its problems. I still have this drivetrain semi-mocked up in my shop if you want some pics let me know...