Will this bolt straight up to my stock catalytic converter?

Yes. This system connects at the cat and runs back—no intermediate fabrication needed. Verify your cat flange matches before ordering.

Do I need to do any welding or modifications to the chassis?

Catback systems like this one bolt together and hang on factory hangers. You won't need to cut, weld, or modify frame points; use what's already there.

Should I expect any fitment gaps where it connects to the cat?

Test-fit the collector clamp to the cat flange before tightening everything down. If the flange is warped or the cat is non-OEM, you may need a shim or gasket—bring those with you or grab them separately.