DiabloGT

well im installing my magnaflow hi flow cat and slp loudmouth this weekend (hopefully), and i think that the high flow cat and stock are different sizes arent they? and im not sure if this will work on obd1, because i think i seen that some will work for obd1 and some dont, and i bought it off of someone in cz28 and i dont think he responds, but im trying to see how i can tell if it will work on my obd1 without any problems. and will it fit right in with the y pipe? becausei think the inlets on it are both 3" but isnt the stock y pipe 2.5"? how would that work?
thanks