Well, since you have no answer yet and maybe you haven't thought of this I'll just throw it out there since I have been in your situation, just needing to get the job going. If you just need to get the bolt "off" if you put a big enough breaker bar on it (think ratchet or flex bar with a long pipe on it) you can probably just break the head off and that will be that... until you want to put a new bolt in there... I am not saying this is a preferred method, but sometimes desperate times require desperate measures. I saw this thread last night and thought someone might have a better answer for you by now. However I am really worried that you say you will attack it from the bottom. Don't pull the car onto yourself!!!! Leave the wheels on. Another idea I saw on a recent thread was an air ratchet with adjustable force. Can sometimes rattle a bolt loose. Of course there is liquid wrench which has worked for me MANY times. But in the end some times the thing just shears off. Hopefully someone else can chime in with any better advice since there probably is some.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra