Cal-Tracs mount to the forward leaf spring mount, effectively as a 2nd "solid" shackle, if I'm not mistaken.
That being the case, I would think not since the Global West SFCs are directly inline with the rear frame rails - if the rear frame rails aren't 'in the way' with the Cal-Tracs, then I imagine the GW SFCs wouldn't be, either.
I just saw this pic that looks like a set of Cal-Tracs along with some GW SFCs (black tube above the forward mount) - the description says it's on a Maverick, though.
Here's another one from Stangnet - '65 Mustang with what looks like GW SFCs (you can just barely make them out) and Cal-Tracs. Here's a link to the thread, which also has a link to more pics (that I can't access here at work):
https://www.stangnet.com/mustang-forums/threads/traction.736697/
Hope this helps!
** edit ** - I also just found this '70 with Cal-Tracs and GW SFCs - looks just fine as well. Plus, it's a pretty dang nice car to drool over just the same.
http://www.classiccarstudio.com/inventory/1727-1970-ford-mustang-mach-1/