need some help

motor was rebuilt and it ran fine when i bought it. I drove it for awhile, best thing running. Well when i parked it for awhile to shave every damn thing, I put this fitler on it. You know the one you can buy at auto parts store. The filter set up with you can put raiser on to rasise it up. Well the pipes that go to the old set up that black thing with the 3 pipes to it, i got rid of. So i welded up the 3pipes that came off the catalitc converter and just welded up the holes on the exhaust manifold witch the other pipe came from. well drove it for like a week, now it want stay started at stops and it feels week, like it dont wanna run. Its sputtering and want warm but like a quarter ways on the temp. But in the mornings it used to drive fine, but now it dont and i drive a good ways in the mornings. So when i get home its still doing this shit. If you floor it it just sputter and get loud. Can any one help me.