Before removing the header you will have to support the roof above it.
Making garage door opening taller.
This means building a wall that spans the entire distance of the opening and is strong enough not to move until you are done.
To put it into perspective if the garage door is let s say 8ft tall make the height of your opening 8ft 1 5 inches tall.
The height of the rough opening must be 1 5 inches taller than the garage door you intend to buy.
That is the side where your truck is to go you can build out on the back side.
Easy to do and it can solve alot of problems.
If you have the room on the back side of your garage you can make your garage into what is called a jack jill garage.
Usually 4 or 8 feet.
One way to do it would be to get wall extensions for the entire the garage and then cut the roof of the garage off lifting it up to the height you want it.
If you are lucky it s in good shape and either made from 2x12 s or even better a glulam beam.