Nat K

hi...i have a 98 dx coupe..and i want to remove the manifold + cat that came on it and replace them with chrome headers and i know ill need a cat but i need more info on the size and if the cat will bolt on to the centre pipe (will they be the same size) and i know it depends on the cat...but i need as much info as i can get please...(ive been lookn on ebay and i know ppl dont like that stuff...but theyre new and im planning on selln it soon anyway just wana replace that) thanks...any help is appreciated