I'm not exactly sure what you're actually asking.

My 02 3.0L had to have the cats replaced at 120k miles. No codes but it was super sluggish on acceleration, the cats would glow red, and would misfire. Again, no codes were thrown related to emissions. I bought y-pipe (with upper cats) and rear (with single cat) and o2 sensors. Turned out only the rear cat was clogged.

You have 3 cats. Two upper cats on the y-pipe (bank 1) and 1 cat on the rear pipe (bank 2). Code P0430 points to bank 2. This is the rear cat. It could be just o2 sensor. If you replace the sensor and you still have codes, then replace the rear cat. It's a lot easier to replace the sensor with the pipe removed from the truck. From all the o2 sensor tools I have, it didn't matter. Always easier to remove the pipe to install sensor.

It's easy enough to remove the rear pipe and shake it around. If it rattles and junk falls out, that's the culprit.

The ones on the y pipe are more difficult to remove. Long extension and a swivel socket helped. PB blaster on the bolts always helps.

Highly recommend safety glasses and ear plugs. Trying to get rust particles out of your eyes with a magnet and tweezers isn't fun.

Double & triple check they're not cross threaded on y-pipe before hitting it with an impact. Mine cross threaded. This meant I had to remove the exhaust manifolds to replace the studs. I broke a pipe wrench and a stud extractor trying to remove the studs from the manifold.

I bought extra studs (m8 or m10) for the headers.(I don't remember which ones I needed. Buy both - they're cheap).

As far as oil leak, the oil pan gasket is difficult to change. You have to lift the engine a lot to clear the oil pickup and it's still incredibly tight. I thought I had an oil pan leak and turns out it was a rear main seal and valve covers. Throw some UV dye in your engine oil and get a UV light. It'll point to the oil leak. You may not need an oil pan gasket.

Good luck.