Tire/wheel size advice
 

I'm restoring a 1970 DeVille Convertible, and will be ready for tires and wheels in the next few weeks. I plan on installing bags on the car and would like to run some larger wheels with low profile tires. Looking at various pics of 70's with aftermarket wheels, I'm thinking of running a 20" wheel. I'd like to get the widest possible wheel, and was wondering if somebody knew the proper offsets and recommended tire height.
Thanks in advance!