First, let me say I was mistaken about nuts. The bolt threads into the exhaust manifold flange, not a nut. If at all possible, be real careful not to break the bolt, or you will have a devil of a time getting the broken bolt piece out. There's not much to heat here, 'cause the exhaust manifold will suck all your torch heat away very quickly. You'd need a major-league torch. Start with the penetrating oil and let it sit for as many hours as you can. An impact wrench from below is probably your best bet for breaking it loose, or breaking it completely, if you try too hard. If you do break the bolts, you'll probably have to drill out the exhaust manifold flange for a clearance hole and use a longer through-bolt and a nut, a PITA. This is not a high-strength application, so a high-strength Grade-8 style bolt is NOT required. I prefer to keep my Zuk all-Metric, so I only have to carry a limited set of Metric tools. If you can find stainless, I would think that it would be less likely to rust in place. Be sure to use anti-seize with stainless, as it tends to gall.
I found
http://www.mmsacc-stainless.com/metric/metric_prod.htm on the Internet, but have yet to order from them. I'm waiting to put together a big order all at once. I'll add replacement exhaust down-pipe bolts to the growing list.
When you do put it back together, a little anti-seize on the threads will make it easier to get apart. The graphite-ed gasket at the manifold-to-down-pipe joint is often bad (mine was). Plan on replacing it.