I had same problem with the mid CAT, I ground the bolts and nuts off and fitted new ones, that saves any future problems too, should be same with backbox
I started 2 fit my Janspeed manifold... then i came 2 the mid cat.... i just ended up putting it all back 2gether, and i'l get the garage to do it, when i get my exhaust done lol
i thought you was sposed to put copper slip or some sort on the nuts/bolts when fitting things like this?? make life easier when you want to take it off
once i feel better - full of a bad cold atm - i'm going to get some new nuts and bolts and cut the whole cat back system off - that way i know it's got mew rubber joints etc.
When I was getting my Janspeed fitted, the place just cut the old backbox bolts and took it off that way. Should be able to drill them out if it needs to be fitted again.