The diameter of the wheels has no impact on the offset you need, just the width. Try doing a search in just the tires/wheels forum for offset. There is a post somewhere that lists ideal offsets for various wheel widths. It has been reposted and linked to in several posts.
Former owner of 94 Bird with DOHC 4.6L swapped from a mark VIII
2011 CTS-V Wagon
1968 Torino was an original 289, C4, fully documented numbers matching car. Now: DOHC 4.6L/T-56 swapped, Cobra intake, IMRC delete, BBK long tubes and H pipe. Dynotech Aluminum Driveshaft, 9" rear with TrueTrac and 4.11's.