If you have ramps, why not drive the car up on the ramps for the front tires and just jack up the rear end and put it on jack stands. I would think that would give plenty of clearance for everything that needs to be done.

I would spray down any bolts you need to take loose with PB blaster even if they were recently installed just so you know they will come loose easy enough.

Why dont you just send me that GMPP cat back and I'll do a long term test for ya and make sure everything is good.