If you have lots to do I would recommend one of these -
http://m.alibaba.com/product/5970303...on_heater.html
Because the heat is localised to just the nut or the bolt it's a lot more effective than using gas. I'm not sure where you buy induction heaters from normally but mine came from my snap on man.
There are a couple of vids on you tube about them if your not sure.