I don't know if this will help, but I stacked a couple 6x6 cutoffs under it with some extra pieces of wood to make up the difference. It shouldn't damage the door and it will hold it well enough to start bolting it in.
Get your self a floor jack with a piece of 1x6 board .Try balancing the door with the jack from a little off center. You can go up and down as needed but dont put any presser on the door if the bolts are tight. I used a small Al-jack
Floor jack trick... with homeade milk crate cradle
Not sure where I read about this ... but the guy used large hose clamps (or drilled for bolts?) to fasten the 1x6 to the jack cradle and fastened a plastic milk case (cheap or free) to the 1x6 with bolts/large fender washers.
The plastic milk crate (cradle) had been cut away on two opposing sides in a bent/curved 'V' shape which about accomodated the profile/cross section of the door bottom. The jagged plastic edges of the opposite sides of the crate had been covered with pipe insulation cut/fit to the contour of the 'V' and duct taped in place.
I never got a chance to try it but it sounded good and stable. Wish I had some pics or a link but don't. :redface:
either buy this, or make one yourself, thats what I did...welded some metal up, got some PVC tubing and wrapped it in Pipe foam... I've used it so many times, works PERFECT!!!