Can't even figure out how to get anything on the passenger side nuts, the cats are in the way
They were rather difficult to remove, even when the cars were new.
Those fasteners are notorious for ends rounding off. Removing a rounded-off bolt (or nut) is rather difficult.
The easiest way is to drop the engine/transmission down along with the K-frame as one big assembly. You have a straight shot at the bolts as well as can add your new Y-pipe (or fabricate one) before installing the assembly back into the car. A bit overkill, yes - but at least it is an option.
Work on the drivers side, first. You most likely will need to cut the old bolts off – which is not a fun job. On the Y that I removed the car (mentioned above), I used a Dremel using fiber reinforced metal cutting blades (and tore up a few of 'em) as well as a Sawzall (with a blade assortment). Even without cats, it was not easy.
If you still can't get to the passenger side fasteners off, and if you have the car up high enough, a person might be able to remove the passenger side exhaust manifold from engine and remove the Y with manifold out from under car...maybe. This is pretty much impossible to do on the drivers side. That said, It will take 3 or 4 hands to man-handle it.
Removing the starter and the oil filter before you start will help, a lot.
Don't plan on reusing the bolts.
BudW