No Cadillac was made without A/C, so you won't find a belt in the official part numbers. Some posts here have indicated it can be bypassed, but it seems to me the water pump gets shortchanged on belt surface when this is attempted. I could be wrong.
To get the bypass belt size, remove the old belt and route a string around all the components except the A/C, measure the result (you might want to subtract an inch or so to give yourself some tension), and take the number to an auto parts store. They should be able to set you up.