bbjsw10 - this thread DEFINITELY doesn't belong in the "Success Stories" forum.
If I were you, I'd be looking to become the first person to do a battery powered block heater experiment. Not with your starting battery though - you'll kill it in short order and I doubt (just guessing) you'll see a net savings anyway with the added alternator load.
If I were you, see if you can source a used 12v truck battery. They'll probably give them to you at scrap rate at a truck service center.