I still have my Grandfathers 65 F-100 that he bought new. The first year of the twin I-beam. I just gifted it to my son upon his graduation from college.
GFaRT, you are right on the money with your analysis of that front end. If you install it properly so the camber is where you want it at ride height, you won't have a problem. It also goes negative in bump which is what you want. Toe wear won't be a problem if all the steering linkage is tight, just like on any other car. You could add an anti-roll bar, (my son and I plan on doing that to Grandpa's truck), and weight jacks on the coil spring were also in the plan to fine tune ride height. Static camber on my Gramps old truck is about 1/2 degree positive, so one plan we had was to redrill the inner beam pivot up as far as we could (about 3/4") and then adjust the truck down with the weight jacks until we are sitting at about 1/2 degree negative on the camber. That should lower the truck about 1 1/2" to 2" with no other modifications. Then maybe add power steering and it will then be good to go.
Folks say that those old Chevy's are heavy. What is heavy on those old cars is the engines and cast iron transmissions. The frames and the bodies are actually quite light. Have you weighed your car? I would have no concerns putting a Mustang II in one of those, and have many times. Mustang II's aren't that light either I might add. All that 5 mph bumper crud, V-8's in many of them, hundreds of pounds of ugly simulated leather upholstery and plastic door panels, and don't forget the padded vinyl tops!
Re-drill the flanges if you want to change the bolt pattern. Not a big deal...