My trailer is a bit newer so it is not a cast frame but this is what I had to do.
I cut a bow approximately the curve of the door and clamped it to the bow. after each clamping I would add about 3/8 material to the form and shave it to zero at each end. Reclamp and check. I had to add about 5 pieces to the original form before the door came back into the frame.
Not sure a cast door frame will take this method but the secret is to take it in small increment.
The picture show the form as first cut while I figured just how to place the clamps.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles