Re: seized, broken bolt Boston
In the 1970's I broke off a distributor bolt and the shop welded a nut on top of it and spun it out, just the way the porsche mechanic suggests. This is routine for a good machine shop or real mechanic, as opposed to some greasemonkey.
I wouldn't try acid, that will eat aluminum and concentrated nitric is sold for cleaning stainless--it generally doesn't care about acids. Also, tends to eat the folks working with it.
Assuming you have tried Kroil or PBlaster? A real corrosion buster, not just WD40? If not, try them first. Soak, vibrate, repeat, the damned stuff is NOT WD-40 it works.
And failing that or the machine shop, i'd suggest freezing them to break the corrosion. A slush of alcohol and chipped dry ice, packed around and allowed to thoroughly freeze the parts, then removed and allowed to come back to room temp (with a heat lamp or torch to make that faster, as long as you don't ruin the temper). Or a CO2 extinguisher, or bottle of CO2 from a gas shop. Two or three good hard freeze cycles will usually break anything free, no danger of detempering metal or starting fires, just some patience and care not to get frostbite from contact.
Loctite also makes a freeze spray in a can now, but I'm not sure how effective that would be compared to really brutal freezing on the parts.