Over the garage door you will likely need a 12" tall to 18" tall LVL header beam. Likely the lower number since its in an endwall and the trusses do not bear directly over it. However there is some framing involved like the double top plate and probably a header that's not really a beam, just a finishing piece of 2x for the top of the door opening.
An 8 foot tall door would be more like a standard size that's "plus" from the typical 7 foot tall, does the truck roof go higher than that?
I would also try to get rid of that old cloth covered wire. Its probably reasonably OK but just something that seems to flake apart at the slightest disturbance. I would read up to learn about the basic design of wiring systems, but I'd find a very knowledgeable friend, ideally a licensed electrician, to be the master, you can be the apprentice so to speak to learn more about the trade. Despite the seeming potential for cost savings this is one area where making mistakes and causing reworks can really outstrip doing it right the first time. And that does not address the safety aspect of it, sizing the conductors and making good, solid connections.