hub centric spacers that bolt on to the existing lug studs and have their own are not a bad idea as long as you get a reputable brand like H&R. Maximum Motorsports makes them too, and everyone here pretty much likens them to the gods of mustang suspension. You want to stay away from the ones you get at like Autozone that simply go over the studs and sandwich between the rim and rotor, those are quite unsafe and can cause you to toss a wheel right off the car. Honestly, for the $150.00 you're going to spend on a good set of hub centrics, you might consider either saving for a nice set of rims all around with 10" wide rears, or if you have bullitts already, just buying some 10" rears to match your fronts. Just my .02. Good luck either way man.
Smokin.....a 1" wheel spacer will add no more stress to the suspension than having a 17x10.5" wheel out back where you once had a 17x8 stocker. it's all leverage.