Depending on how much rust there is on the nuts and bolts, they are a Beeatch to get off. I sheared off one bolt off the passenger side header - the one that is right behind the O2 sensor - if you can get that off it makes it easier to remove the nut. But to answer your question, they are not bolts but threaded studs. The end that threads into the header flange is of a wider diameter than the other end that the H-pipe connects to - so the size of socket that fits that end won't help you. There is also no nut on the other side - it simply screws into the header flange. As a result, if you really really really want to replace it, you are going to have to use a grinder, cut the bolt off flush with the header flange, drill out the remainder, tap it and screw in the new stud. I can pretty much guarantee that there is no way you will be able to screw out the existing stud - there is nothing to get purchase on to unscrew it, and it likely is rusted solid.
Hope this helps
Howard