Purchasing a bug sti and it will need decanting how do I know the cat will fit to the stainless cat back it already has??? I'm not worried about it fitting to engine side but wether the middle section will for example it's 2.5 and the decat is 3inch! How do you even get a precise measurment??

 

you buy the correct one.. a 2.5" if that's what is already on it although I am lost with your question... your decatting and replacing it with a cat?