Re: well it happened..
my 98 has a pre-cat and a cat after the y pipe. it has 2 02 sensors in front of the pre-cat and one after the cat. replacing it shouldn't be a problem, the new converter is rated for a heavier gross weight and bigger displacement engine as well.
As far a legality, im replacing 1 cat with another, the original has over 325k on it and PA inspection wont pass this one because it is welded as a repair.
the line of thinking on building the new system was to pick up more low end like long tube headers, but maybe im wrong. no big deal, by retaining the stock y pipe with the flange, a stock type converter can be bolted up to it incase there is a legal problem
98 ext cab, short bed. 4.0 5 speed Hurst shifter, 3 inch bl,T bars cranked, Adda-leaf springs,James Duff traction bars, Pro Comps 10.50x31x15, Black wheels, AVM hubs, Shiftster equipped, nerf bars
Mustang rear disc brake swap, electric fan conversion no more headers