Exhaust Manifold busted nut

I have an '08 base 4cyl taco that needs a new cat / O2 sensors. When I unbolted the cat from the exhaust manifold it appears that there is a nut that is welded to the exhaust manifold that must have disintegrated. I asked Toyota if they had this nut but they said since it is welded to the manifold that they don't sell just the nut. The parts guy at the dealer recommended just finding a nut at the hardware store and use that. Well the spring bolt that connects the cat to the manifold is tapered at the end so I'm not sure that will work so well. Has anyone encountered this issue before? recommendations?

Btw, the attached pix are from my other Taco. Same year/make/model...

 

Those things are the worse.i bet the nut is super fine thread

 

New bolt / Old Spring / Old bolt

 

Where did you get replacement from ?

 

Dealer. They just didn't have the nut...

 

Grind the rotted welded nut flat and use a regular nut.