Compare a Mustang to a Mopar or GM - with the mustang the whole front end is all thin stampings and sheet metal, there isn't even a real crossmember, just a piece of tubing bolted in there. At least GM and Mopar have a thick, strong crossmember. And whatever carries the front
suspension loads in the Mustang, be it the frame rails or the shock towers, it all runs into 18 gauge body sheet metal in the end.
I'm convinced early Mustangs were never intended to last very long and probably never intended to do road course competition, drag racing or any other sort of high-stress stuff. They were cheap cars, plain and simple.
Whatever front
suspension you choose is going to be a compromise but, as stated earlier, the MII is a great choice for packaging, performance and budget reasons.