exhaust fittment

[IrishBeatdown]

So, I had my exhaust rewelded so it hung correctly about 2 weeks ago. Today, I was driving and I suddenly had smoke coming out of the engine compartment. Pulled over, popped the hood, and saw the downpipe was leaking. Jacked it up and looked underneath, turns out the idiot from the exhaust shop didnt bolt up both bolts on the downpipe, one was missing. The gasket was fouled, had to throw it away.

My question is:
The SS o2 housing is 2.5"
My punishment downpipe is 3"

I ordered a 2.5" and 3" gaskets from vibrant on friday. Which gasket should I use, the 3", the 2.5", or should I butt them up together and put in both? Just want to make sure I don't have this problem again.
Thanks!

 

[FIBA M - EC17PSE]

you can use the 3" if it still seals correctly or the 2.5" as its not going to harm anything its still the same as your o2 housing so will flow the same as before