Duster and Demon owners

Interesting! I'll have to check the set of '72 doors I'm going to put on my Duster.

I haven't seen that before, but it's definitely a stress crack because the door is flexing there. And it makes sense that it cracked through one of the mounting holes because that's where the least amount of metal is.

Access to that space is going to be pretty limited, but what I would do is get like an 1/8" thick piece of flat stock that's the same height as that flange. Then drill the holes to mount the whiskers, and spot weld it to the back of that flange. Put a spot weld between each set of mounting holes for the whiskers and you should be good to go, you won't be able to see the flat stock behind the flange with the door assembled or the spot welds with the whiskers in place.