What was the problem with the MII from in the 64 Ranchero? The only thing I can think of is it's not much of an improvement. the suspension design isn't that much different from the original Ranchero design except for the spring placement. The high spring on the original design is very roll resistant -- much more than the MII. You'll need a stout roll bar with the MII suspension whereas the original really didn't need one for normal driving. The real benefits of the MII design over the original is the MII has built in anti-dive (shouldn't nose down as much as the original) and parts availability. Parts aren't real hard to get for the 64 Ranchero suspension though.
The MII is about as much of a "universal" suspension as you can find, but it's not a big improvement over everything. It's great for cars without an good independent suspension or one that's hard to find parts for. Anything else you're compromising the orignal engineering that went into the car.
In this case (64 Ranchero), I don't think there are any real advantages to the MII design at all, unless you need to get rid of the spring towers or you're building something that you need to change parts around a lot on. Maybe a road racer that you'll be setting up for different tracks and need to change or adjust the srpings. If you just want to improve ride and handling, 10-20% stiffer progressive rate springs to replace the originals makes a world of diference. Coil Spring specialties will wind the coils to your specs for the same price as stock replacements, just takes 2-3 weeks lead time.