My plan is to replace my SUPER leaky factory exhaust with a custom bent "downpipe back" system. My only question is, do I need some sort of adapter for the 2.25" exhaust tubing? I was thinking I could use the factory cat flange and just weld the pipe to it, but will that work even if the pipe is bigger than factory?

Sorry if I don't make sense... Exhaust isn't my thing

 

yes you can weld to the original flange. if you need an adapter for it, the muffler shop can bend the end of the pipe with the pipe expander on the tubing bender machine to match the size.
a muffler shop can do what you need.

 

While you are in there put a header in... makes all the diff in the world for low end power

 

I'm gonna wait on headers since a set for the veezee runs about $700... Eventually it'll get 'em, but it's not in the budget right now. :/