here's a quick list of tire sizes for the f-body. with these tire sizes, there is no need to recalibrate the PCM because they are all the real close to the factory height.
245/50-16 = 25.7"
245/45-17 = 25.7"
275/40-17 = 25.7"
315/35-17 = 25.7"
245/40-18 = 25.7"
275/35-18 = 25.6"
285/35-18 = 25.9"
315/30-18 = 25.5"
335/30-18 = 25.9"
245/35-19 = 25.8"
285/30-19 = 25.7"
295/25-20 = 25.7"
what you do is shorten the sidewall of the tire to accomodate the larger diameter wheel. all will be same height, just in different widths. then, depending on the offset and width of the wheels you buy, will determine if you need to 'clearance' the inner fenderwell or not. then, depending on how much you lower the car, and offset/width of the wheel, will determine if it will hit the outer fenderlip.