If penetrating oil isn't going to shift them then the options and combinations are:

Heat
Nut splitter
Angle grinder

Pay the local fitter 50 quid.

 

much easier if you can take the back box off with the cat then undo the hard to reach ones that way using a heat gun/blow torch then smack them out with a hammer and replace them with new ones

Blow torch was the only way they would move for me

 

With a blow torch mine took 10 mins for all the bolts.

 

If you have a friend with a pit :dell: it isn't too difficult to get some decent leverage with a spanner and rachet!

 

I found when I tried that it wasn't the fact that the bolts were stuck and you needed extra leverage, but rather that the nuts were coroded and wouldn't allow a decent fit with any tools.

 

remove any excess rust, then use a big hammer and prepared with a punch and vice to get them out the socket once you've undone them ha ha!

 

Be cautious of hitting the nuts too much, the impact could cause cracking in the cat and ruin it, if they will not come undone normally or with the help of heat best to cut them off and replace with new stainless steel items.

 

If only you had said nuts instead of bolts....... I would have said head first into a wellington boot and off with them