Shelby drop and back spacing of wheels

My dad's looking at doing the shelby drop to his 66 GT fastback.  The car is currently all stock with the exception of a larger sway bar.  The car has 14x6 keystone classics on it and we'd like to keep them, however, the backspacing on them puts them pretty much flush with the wheel well.  I'd like to two wheels with the right back spacing so that we don't need to find a whole new set.  

So, what's the best back space for 14x6 wheels and the shelby drop.

Thanks!