Is there not a reading on the inside of the rim? Maybe not. I will tell you that those FWD GP's have a ton of backspace where you would need maybe 2+ inches of wheel spacers to come close, adding the need for long wheel studs also. 4" backspacing on an 8" wide rim would be ideal for our cars. Also, that bolt pattern (5X115mm) is close but I don't recommend running them. They WILL bolt on the Buick but will they stay on at 100 mph? I wouldn't take the chance myself.
I think you better keep looking.