First, always use Reverse drill bits when trying to remove broken bolts.
It drills like a regular drill bit but also has the added advantage of unscrewing the bolt if it catches, also the vibration of drilling helps that along
Many broken bolts unscrew using just the Reverse bit.
Heat probably isn't an option because of the location, so about all you can do is drill them out.
It can take some time, use lots of oil, start off with smaller bit size to get some material out of the center and then move to larger sizes.
And yes if it is that rusty then you may have to drill out the hole and retap it at larger size.
Heat kills drill bits, dulls them, so use oil to cool bits constantly
Last edited by RonD; 04-09-2016 at 09:41 AM.