Which size exhaust to go with?

All headers collect into a 3 inch tube right?

so anything else uses a reducer off the get go, no problem.

will a 3 inch exhaust system from header to the tail pipe fit under an a body without any problems?

specifically I am looking for my 68 dart.

furthermore, with my 360 engine, do I need 3 inch exhaust? will a smaller diameter be better or worse? will a 3 inch system be worse?

there is a reason for the differing sizes, what has everyone else used?