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.
If it's just the bottom of the towers you can patch it alot easier than replacing. They are nice thick steel, and very easy to patch, You can weld in pieces and grind them smooth and it will look like original. Oh and make a nice drain hole in them so they wont rot again, the original design sucked.