Before removing the header you will have to support the roof above it.
Making garage door opening taller.
The height of the rough opening must be 1 5 inches taller than the garage door you intend to buy.
Easy to do and it can solve alot of problems.
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.
If you are lucky it s in good shape and either made from 2x12 s or even better a glulam beam.
That is the side where your truck is to go you can build out on the back side.
That made my door height two inches taller.
I removed some siding and and re did the trim on the top of the door and then recovered every thing with aluminum and installed new rubber garage door sill around the opening.
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.