I have installed several Heidt's MII front ends and love them. The factory welds on them look like they were done by a robot -- there is no way for them to be better. The way the Heidt's kit reinforces the frame rails, there is no reason to worry about the different loading except under extreme track conditions, in which case tubular reinforcement inside the fenderwell from the front of the frame rails running up to high on the cowl completely fixes everything.
I have seen and done all of this first hand, unlike most, if not all, of the detractors of these systems.
The ride and handling are great for a street car. Many people will scream that you can get better handling from other
suspension types, which is true, but you're only ever going to see the difference at 9/10ths or above, which very few people here have ever seen. Most Mustang owners desire a nice, comfortable ride, handling that is much better than stock, crisp steering, and nice disc brakes. The Heidt's MII kit gives you all of these.
To be fair, I have never installed the Rod & Custom version, but have only heard good things about it. There are aspects of their design that I like better than Heidt's, but they don't reinforce the frame rails nearly as much.
You can find plenty of pics on my website. Feel free to email or call if you'd like to know more.