Hey yall, just bought this Versa from an old couple for me and my girlfriend to drive around and the “Service Engine Soon” light came on so we took it to autozone and ran the ecu for codes and got catalytic converter emissions below threshold, so unfortunately we can’t get her car registered until we get this conundrum figured out. Instead of assuming it was the downstream O2 sensor throwing a false code, we got a new catalytic off of eBay for $50 because the sensor was around $150.   
Got the catalytic in the mail yesterday and went to go take the old one off, but got stuck when I snapped a bolt off the front end of the catalytic where it connects to the exhaust manifold/header. Mind you I did my best to get some P-blaster up in there and break up the rust bond on each bolt and it worked for the back end but not this front end. Now I’m left with two smooth bolts stuck to the end of the exhaust manifold and no tool or power tool to fit up in there and get it out. Any suggestions??? Considering tap welding a nut on each bolt and seeing if it gives me leverage enough to break them free, but I’m worried they’ll snap off and still leave the portion on the backside.   
TL;DR  
My girlfriend can’t get her car registered until i figure out how to remove these rusted bolts and throw on a new catalytic and new 02 sensor.   
Any help or suggestions are much appreciated 🙏

2009 Nissan Versa 1.8L - Catalytic Converter - 150k miles - Automatic trans