There was a show on Mythbusters about that very topic. Ended up that they couldn't light a room filled with gasoline vapor very easily.

Thinking about it a little more. Probably the best solution is to open the dooor and let the vapors out.In that case you would want the heat source which would heat the shop the fastest. Assuming that the shop was already warm before opening the doors, it wouldn't take very long at all to get it back up to temp with a forced air furnace. Perhaps ther is something faster but I can't think of anything right now.