my only problem is that the 6 inches gradually reduces as you travel across the front of the door opening so it eventually educes to a perfect fit. The 6 inch gap is only in one side

See these pics that i took. As you can see on he left hand side

is where we have the 6 inch gap. To make matters worse we have a downward sloped driveway going into the garage so we will need clearance to get into the garage and room for the garage door rails

It needs to be demo'ed. Graded properly, so the water drains down the driveway, into the street.

What's your budget?