Im sure the rules are if the engine is out of a car that does not have a cat then you do not need it in the conversion.. well it might have changed... the guys at the exhaust shop asked me when i got the 3 inch on the sapphire if i wanted a cat and i said well the car never came out with one and they said oh ok no worry then and just fitted it without it... anyway then they asked the same question when i got the exhaust done for the mk1 they never put a cat on..
anyway you obviously "have one" so you wont get caught hahaha..
anyway you have to drive it to get caught too so hurry up![;^)]