Scooby Jo

Hi, I bought a Pro Drive stainless steel back box - it came off a 05 STI - I have a little 02 GX. Sold in good faith by fellow Scooby Net member so no quibbles there. I know the downpipe is a different size but I'm sure that can be sorted. However, took it to my usually reliable mechanic who informed me it would wreck the engine on the car if I fitted it.

Before I try and find someone else to fit it and faff around with the fitment, could anyone let me know if I am wasting my time and money? If I am I'll sell it on and put it down to "experience"!!

Cheers
Scooby Jo