Picked up a set of 15 X 7 Buick rally wheels off of a '69 Skylark for my 69 Sport Wagon. They look nice, I have since picked up a set of 4 tri-shield center caps for the wheels. They are chrome with a color insert of the shields on a black background. These look like they are too big or will be a struggle to put on. What is the correct diameter for the center cap and how in the heck do you get them in place?