Changing the stock 2 1/4" exhaust to a 2 1/2"

[c_mcb03]

Looking into how I can move my 2 1/4" stock exhaust on my VX Calais into 2 1/2", but I wanted to check how the 2 1/4" cat converter flange would mount to a 2 1/2" cat-back.
Will this cause an air leak because there will be a 3mm difference on each side? Is there a type of gasket I can buy to seal this, or will I change it to a 2 1/2" cat?

 

[Aus78Formula]

Sounds like a job for the local mechanic or exhaust shop. Try to think of other trivial work that needs doing while up on a hoist, always a bonus even if a simple oil change you'd normally do yourself.