Backspace question- 18" wheel on 68 Camaro

[DWC]

I'd like to get confirmation on the specs for some wheels I'm interested in for my 1968 Camaro.

I have Ridetech coilovers, 4-link, Strong Arms and Tru Turn kit. Fenders and wells are all stock. Will the following specs work?

Front- 18x8.5 et15 (about 5.4"- have to use a 10mm spacer) with 255/35/18
Rears- 18x9.5 et15 (about 5.8"- have to use a 10 or 15mm spacer) with 275/35/18

Also, should I go with a 35 or 40 ratio tire?

Thanks for your input.

Daniel