Excuse my ignorance, but what am I looking for as far as rim specs when buying a set that will fit over the granda disc setup? I know stock rims wont fit, but what specification will let me know that an aftermarket rim will fit?
You need a 15x7 min. As for back spacing, the 3.75 like on the basic Torque Thrust will work in front, but I found it rubbed the rear fenders in the rear on my fastback. So you might need to go to a 4" back spacing. Unfortunatly, every car is a little different. I was lucky, a friend had a set of 15x7's with 4" back spacing from a Ford Ranger and I was able to test fit them, maybe a wheel shop can help you out.
I ran 14x6 rims with 3.75" backspacing and 205/60-14's on my Granada Swap for a couple of years. The issue is primarily that the original Mustang styled steel wheels have a smaller center opening that hits the snout of the Granada calipers. If you want to run those wheels, you have to get the Granada caliper snout turned down by a shop on a lathe.
I'm not aware of any other wheels made for disc brakes having a problem with interference.
Ditto what JamesW said. It is not the 14" rim that is the problem it is the fact that the older rims had a smaller center hole because the older drums/rotors had a smaller centering hub. You can use later model 14" steel rims or any aftermarket 14" rim or like James suggested you can have the rotor hubs turned down on a lathe at a machine shop.