I am looking to put aftermarket wheels on my 66 Bonneville. I am trying to figure out the correct size/offset. Has anyone gone through this yet?

Yes, your best bet is the 15 X 7 with a 5" back space...

I would go with the chrome reverse or Pontiac Rally rims. The stock style steel rim is hub centric and is a much better rim in every way then the aluminum lug centric rim.. You want a rim that the center fits snugly against the center hub and can accept clip on balance weights on both ends..

I've had 8 lugs then American Racing rims and the chrome reverse I have now are light years better in every way..

The Pontiac Rally rim looks great on the 65-66 B body....