I would plumb in gas and look at installing a gas heater well above the
floor. One option many use is a second hand, or salvage residential
furnace. These can be bought reasonably from construction salvage
operations, heating contractors or via want-ads. If you have overhead
space, mount the base of the furnace a couple of feet off of the floor -
this gets it above a lot of the fumes that settle. In a smaller garage, you
might not need much in the way of ducting. I purchased a good forced air
furnace several years ago for $25 but most are $100 or more. If you buy it
from a contractor it should be inspected.

Another, higher cost, option is an overhead gas furnace like that seen in a
lot of auto and wood shops. BTW, if you have South exposure don't forget
windows - solar gain is cheap, cheap, cheap.

Might run this past the local fire department but I believe it will be ok.
I agree with others that wood is not your best approach.