Rotten egg smell in my experience has always been a cat.
Especially with 140k miles that you have on this hulk.
Shop ebay for a couple aftermarket highflow universal cats and call it done.
Does yours have the 3.9L v8 tbird motor?
I never trusted these cars for some reason, its a shame that ford didn’t put the quality into this thing that the markviii’s have.
Its basically a piece of shit jag as far as I’m concerned.