Yes you are correct the size is 255/50ZR17 not 18.
I do not think that any tire shop will be willing to change the original tire size to a new narrower tire, even if the Caprice uses different narrower size tire? I will check on that when I need to install new tires, they only last 40-45k miles. If I want to keep the original rims there is a limited range of sizes I can choose from?
I have also replaced the front anti-sway bar bushings hopping that it will help, it help in my other car, but there is no difference in the Impala.
Thanks,
mark