In exterior size, is a mitsubishi TD05-18G the same *SIZE* as a TD05-16G or even a 20G???

I realize that the bolts to the down pipe will not be the same being that the 18G has 3 holes to screw the down pipe on and I believe the 16 and 20G has 4 holes. Which, if that's the case, I can always swap the exhaust housing from my 18G to the 16G and go with that.

ANyone got any thoughts on all of this?