I snapped off the threaded collar on the catalytic converter for the front oxygen sensor and am now trying to replace the whole catalytic converter from flange to flange.  How do you get the top bolt off of the front flange between the cat and the header pipe?

Without dropping the Y-pipe? Sneakily.

 

I had to remove the small heat shield section that covers the notch of the Y. It's a PITA to get out of there, but will give you access to the flange bolt with a socket and straight extension.

That was my thought.  I will have to grind off screws from the heat shield as they are rusted beyond removal.  Is it best to drop the Y-pipe?

Pull the whole assembly off the car and just have an exhaust shop weld a new bung on. Much cheaper than replacing a cat.

 

Fairtax4me - I like that idea.