Go to Exhaust Plus at Stepney, get Pete to throw it over the pit and see what will fit in the small gap.
You could always go under the diff and get him to flange it just before the diff, so if you ever need to get the diff out, just unbolt the rear section.
This way, it will give you more length to play with, without having to go over the diff, so to speak.
He know's his shit and I wouldnt go with anyone else for an exhaust system.