has anyone ever cut a 02 indy 500 exhaust in half and gutted it was wondering if it was louder and did anything to performance?

lots louder ,slightly faster ,need to rejet.

if it has a 340 main jet what would u re jet to?

You would have to figure that out by trial and error start with a 400.
gutting it is probably a wast of time unless you are drag racing.