Having restored old cars (most with at least minor rust problems) all my life, I've busted a few bolts off. Here's a few tricks I've picked up over the years.

1) 1st, if you expect trouble, use penetrating oil early and often - WD40 will work too, but not as well. Use liberally. Soak from every angle you can get to. Soak over a couple days if you can - reapplying if necessary. Even if it doesn't look like it can get in the threads, it will. Rust is highly absorbent and draws it in like a sponge.

2) Heat is your best friend. Get a map gas torch. Heat the area so the two metals (the bolt and the metal surrounding the bolt) can expand. Usually they will expand at different rates such that you can break it loose.

3) Heat and cold. (never tried this one but heard about it) Heat the area as noted above, but shoot the bolt with compressed air from a keyboard cleaner held upside down (shoots ice cold air out). This will contract the bolt. The flip side of this- if the part is off the car and you can put it in a freezer - freeze the part then put heat to the surrounding part. This method is often used for getting old bearings out of hubs.

4) Last option: If you break the bolt off, don't worry. Now you have better access to the threads. Douse with penetrant and let it soak. Get a set of EZ outs (weird drill bit looking things). If you still cant get them out, drill out the old bolt and re-tap.

Remember preventive maintenance too. Whenever you replace bolts, except those that you want to use thread lock on, put a little oil on the thread to avoid these good times in the future.