generally with these cars, 4" back spacing is a safe bet
the build quality is questionable, and some cars are comfortable with more BS, and others with less BS
it also depends on the type of tire you chose. A large sidewall tire will tend to bulge out more than a low-profile type tire.
for example, a 275/60/15 tire with a big sidewall will bulge out more than a 275/45/17 low-pro tire
generally, on Regals on Montes for whatever reason, I've seen 4.5" BS work comfortably with a big tire. The GPs and Cutlasses have clearance issues (GP with frame rubbing, and the Cutlass tires rubs on the low fender skirt)
in the front, 4.5"BS should work okay, you might notice some rubbing on the swaybar & frame at full lock turns
tires, depends on the look you're going for
if you like the staggered 15x7 & 15x8 look, I could recommend a 215/65/15 front, and 275/60/15 rear
in the front, you could go as large as 235/60/15
in the rear, you don't want to go larger than 275/60/15... and really a 255/60/15 is a safer bet
here's a picture of my car with 215/65 fronts and 275/60 rears on Chevy rally wheels
(these are 4"BS wheels, I had to use 1/2" spacers and raise the height of the rear)