It's not outrageously difficult, but it's a very big job. Engine out, fenders off, front suspension off, drill out a gazillion old spot welds, do some serious hacking, and weld new ones in. All this while keeping all of the measurements and angles striaght so all of the above line up right again.
At this point, if you're not interested in stock restoration (judging by your handle), you might as well consider putting a Mustang II-type front suspension kit in. It's the same amount of work or less, and your car will ride and handle way better.