I haven't looked under my dang car yet, but aren't factory cats typically 2 part ( a pre catalyist, then the real cat) and after market a one shot deal? At least I remember my 88 GT cat pipe having 4 little cat like things. Anyway, anything's of course 'doable'. But seeing how aftermarket cats typically go for $100 a pop anyway, there's not too much money to be saved by having new cats welded in, compared to shelling out $300-400 for a high flow cat pipe.
Edit: Besides, if your new welded cat pipe fails emisions, you wont have the stock one to throw in.
Thanks man. No, it was a piece of cake to put in actually. Turned out to be just a 3 beer job (like 2 hours) . Just have to be careful to not scuff the paint both when drilling the hole in the plastic piece and when actually sliding the fuel door assembly in place. They recommend using masking tape on the paint around the door, I just draped a towel over the side and was careful. Man, it sure would suck to put in a nice piece like that in and scratch your paint in the process!