OK, so that isn’t as bad as I thought it might be. If this was my build, I would be welding the exterior door flanges on.
First step is to cut and pre-shape the flat strap. I use a minimum thickness of 3/16” about 2.5” wide. Mark a line down the middle of each strap and drill 4 or 5 1/2” holes down one side of each strap so that the edge of the finished hole is about 3/8” from the centerline. If you have (or know a friend with) a metal roller, use that to roll the side straps down to the tank contour. If you don’t have a roller, tack the strap to the back side of your tank and use a BFH to tap the strap into shape. Once flanges are formed, grind door edge flat, position strap so that half of it is on door, then tack them in place using plenty of c-clamps to hold them in place and flat against door. After straps are tacked down on inside edge (edge laying flat against top of door) weld the strap to door exposed inside the holes and then weld holes shut. Stitch weld inside edge and remove clamps.
Repeat process on inside edges of tank and then fit door to tank adjusting as necessary.