There is additional steel above the door. A thick header plate and three ribs. Bridge all three ribs to avoid deforming the sheet metal. You'll want to remove the cover from the interior to measure and align your holes to match the heavy steel.
It may be necessary to attach an adapter (unistrut, 2x6, etc) to the exterior bus, using the interior alignment you have. Then mount the hangboard to the adapter. Spring nuts or lag bolts, respectively, though the factory holes in the hang board.
Use adheasive/sealant, such as Dynatron550 in addition to mechanical fasteners. Dab the rivets/bolts and one face of the material where the two are squeezed together. Both to defend against leaks and aid the shear strength of the fasteners.
Any sample pics of the hangboard style you will use? Rings, Bands, etc...