would a 2000 si stock cat back exhaust bolt right on a 1998 honda civic ex? or would i have to do some welding? also would my 98 ex stock muffler hook right up to the si exhaust or would i need a 2000 civic si muffler swap aswell?

 

not sure about the cat

however, the muffler (axle-back) exhaust portion will fit no problem.

 

well if its a 2000 si then its a b16a2, in which case if you bought a pre-made catback it would be a different model than your EX's exhaust, although a muffler shop could probably make some slight modifcations to make it fit pretty easily. But i don't believe it'd be a direct swap