The width that will fit will depend on a number of factors. The most important one is the backspacing of the wheel. If it is 4 inches, you can easily fit a 245 in the rear. I have run 245 50 16's on my car with no issues, but I did find that the paint on the frame seemed to be stripped away in places. I figured it may be dirt blasting the frame due to how close they are, or they may be rubbing slightly due to bushing flex (although I have neither heard nor felt any rubbing). That is another factor to consider. If the rear bushings are wasted, the body could shift on the axle and allow contact. Then there is the issue of how square the body sits on the frame. Most of the times, these cars do not sit evenly from the factory and the body is shifted to one side or the other. The solution is to loosen the mounts and shift the body. This is easier said than done if the mounting bolts are rusted to their fasteners in the body as you could shear one off and have a bigger issue on your hands than tire rub.
I run the same springs as you on my car, but run 245 50 16's on 16x8 in wheels with a 4 in backspacing all the way around. I did extensive fender trimming up front to stop it from rubbing though, so be warned about the possibility of interference at full compression up front should you decide to run 245's up front. In all, I spent 30 hours cutting, welding and reshaping the fenders on my car to make everything happy to be together. If you don't mind the rub though, you can get away with it. Just remember that the Cutlass has the WORST wheel arch shape of all G bodies for running wide fronts. Stock, GM never ran larger than a 215 65 15 on a Cutlass, so it was not an issue. The problem arises when it is modified.
