I agree with Mike that 12' is going to be tough for an overhead. By time time you add the rails, the hoists, and the slings, your timber will be at 8 or 9'. But it could work. I made my own in my first shop. Definately not OSHA approved!
Picture is 10 years old, so sorry about the low quality. I know it doesn't show the bridge very well, but it's the best shot I have. The travelling bridge is made of two short I beams, and is suspended from the fixed long beams.
How about loading and uloading the truck? That turned out to be the bigger problem than moving timbers around in the shop.