I still would insulate the garage up to house specifications (you seem to be almost there) and then run some duct work from the furnace (exactly how will depend on your setup, my house would be tough but my sister's house would be simple). Unlike what I said earlier, I probably wouldn't just leave the house door open. :D but I'd seriously consider making the garage just another part of the household furnace system. think about it, if you let your garage freeze, then kick on the new secondary unit, it'll take a while to get the air reasonable, but it'll take forever to heat up the truck and your tools (and it'll stink the entire time). And you'd waste a ton of fuel every day you went out there to work, even if it was only for an hour. Especially for this winter, where the garage is destined to be a project room and not a car port (meaning the garage door is likely going to remain shut), I'd infinitely prefer that everything just be warm, maybe not 70* like the house, but definitely not 20* with a hot flame burning whenever I'm out there.
choice is yours though.