Thanks for all the advice guys... I was able to get these bolts out
eventually, without the torch, but all these replies have got me
thinking.  my original question of whether to run the car for awhile
to get the exhaust all hot was kind of along this same train of
thought, and i was cautioned that if the engine wasn't cool, i risked
damage to the aluminum threads as i tried to force the steel threads
out.  now i have heard many times before of the "hot wrench"... so now
should i refine the answer to my original question to be: it will do
you no good to run the car all day because it will not get hot
enough--what you need is an oxy acetylene torch.  this will not cause
you to strip the aluminum threads and will not cause any warpage of
any metal due to intense differential levels of heating across a small
area.  it will only help to get bolt out.

now if only i had that torch....