Is the rear O2 located in the middle of the cat or behind the cat? If its in the middle I don't think the leak would cause the code, but if the sensor is behind the cat then the leak is possibly the culprit.
I used to get 0420's & 0430's in all combinations of the two -sometimes one or the other, or sometimes both at the same time. The PCM's do not only look at signal strength to set a code.

The 02 sensor is after the cat.

UPDATE: jacked up the car, and undid the clamps that connect the midpipe to the overaxles. There was a .5" gap between the 2 pipes on the passenger side, and barely any on the driver's side where the problem exists, maybe 1/8". I doubt this leak is the cause...

Also, just spoke to a rep from Bama, they said there's not much they can help with when it comes to emissions inspections. They recommended anti-foulers and that's about all the help I received.

Try doubled up non-foulers, they might just do the trick for you. They wouldn't work for me but I am cammed & boosted. If they work, its an easy cheap fix.

Definitely going to try either those or J dungs [if that's the correct name]. A friend told me about them they're basically an L-shaped non-fouler. Anyone know the proper name? Anyone know What size would fit for an 06 mustang? Found some on jegs.com for like $5, they have 18 mm and 14mm.

Unless anyone has a recommendation on where to get them

Well good news. I did 2 things recently: swapped the 02 sensors from bank to bank to see if one of the senors were faulty, and fixed my exhaust leak (somewhat). And I say somewhat because I can still smell exhaust while idling at a red light, just a little though. Anyway, I guess the .5" gap in between the midpipe and over axle on the passenger side, as well as the small 1/8" maybe on the drivers, was the cause of the p0430.

Congrats and very interesting.

It's strange. I guess the slightest change of temperature or exhaust amount in the pipes made the difference.