CAT exhaust problem

I have a couple of issues. In attempting to resolve prop vibration I have encountered rusty bolts probably fitted in japan to replace original studs on the back of the catalitic converter flanges, which couple to the after market stainless exhausts. I have managed to remove two of the original nuts from the studs, but the other two have been replaced with these bolts which have now sheared. What is the best way to remove these? One has sheared flush with the CAT flange, the other about 1/4" above. Do I grind flush and try to drill them out? There is not much room behind the flange to grip the end of the thread??

Also I intend to replace the centre bearing. I have an apparently good 2nd hand prop, but unfortunately it was sent in two halfs unmarked. Is it better to dismantle the existing prop and swop centre bearing or risk 1/2 and half? Or are the props balanced as a whole??