I installed the bosal cat-back on my car....
YES I reused the stock bolts...for the simple reason that to get them out of the cat you would just about have to remove it to knock them out....they are press fit into the flange.
even if the stock gasket will work with the system...which I doubt....the answer would still be NO, get a new one. The reason why...is the stock gasket is a "spiral wound" gasket...and are intended to be used only once...once it is crushed (used) its done....you can't resue them and expect them to seal.
The gaskets that came with my system were of the standard flat perferated metal backed exhaust gaskets....worked fine, still working fine....
and I would be thinking you would need TWO gaskets...one up front....and one just in front of the axel at that joint.
the easiest thing to do....take a piece of paper or cardboard....and a pencil.....put the paper/cardboard over each flange...draw an outline with the pencil and then head off to the local parts house and just tell them you want an exhause gasket and here is the flange it has to work with. and remember the hole size in the pipe...no need to put an restriction in the line with a gasket where the hole is smaller than the pipe.
