I am Try to purchase Some new wheels for my uncles 78 Buick Riviera and I am unsure what size wheels to get. He wants the widest wheel possible as the car is receiving a  big block transplant in a few months and will need all the traction it can get.
What are the widest sized rims you can fit under the stock wheel wells? Also the car has adjustable air shocks in the rear and can raise up a few inches so how big can i go in the back?  This is for a Christmas gift and I am eyeing a set of 15 inch Billet Specialties street lites for him.   Any help would be greatly appreciated as i dont have a clue about wheel sizes and i suck at math. Thanks