18x8 coys on 79 cutlass

[Pro-Touring79Cutlass]

im looking at 18x8 coys c-5s on my olds ,or 17x8 i had posted before for the 17x8 and i had been told that a 4.5 inch backspacing would fit ,i was looking at the coys and the 18x8s are only 50 bucks more than the 17x8s what backspacing would i need and any sugestions on what size tire to run on the 18s