I had the same exact problem.
I have a 67 Shelby clone with all Global west parts, and a TCP manual R&P.
finished the car about three years ago, and when ever I get around 60mph Id get a L-R shimmy of the steering wheel, and when I went faster , it got more violent. Everyone kept saying your tires are out of round,For the past three years Ive been replacing parts, went through 2 sets of tires, 2 sets of wheels , 8 balance jobs, 3 alignments, new brakes, rotors, tie rods, ball joints, etc. etc. etc. you name it, I replaced it,drove me crazy.
If you go to the TCP website theres a link for Tech. support, a guy named Lino runs it and was very helpful. We emailed each other many times a day on different things to check and test and adjust.
I eventually found my problem myself. I found Pro-Motorsports.com and read about bump steer and the kit they offer. In a nutshell, From the factory early mustangs have some bump steer issues, but when you lower a mustang, or lower the upper control arms,and install an aftermarket rack and pinion, you change the geometry (angles) or the upper arms and tie rods.
I installed a bump steer kit, that lowers the outer tie rod end 1" (where it connects to the spindle) Did a temporary alignment job in my garage.
and no more shimmy at highway speeds!!!!!! I was finally able to unleash the beast.
I still have to get the car professionally aligned. The only place that knows how to align these cars is Marlos Alignment in chatsworth CA. And Ill be making the 2Hr journey soon.Make sure you Align it to TCP reccomended specs.
TCP should address this issue in their instructions and make a bump steer kit of their own.For some reason they dont.
Im not saying this is your problem, but from what it sounds like, I experienced the same exact thing, and by installing this bump steer kit sloved it.
good luck