Back Spacing for 35" tires

[KrustyCraftsmen]

I finally up graded my suspension lift on my truck to a 6" RCX lift in conjunction with a 1.25 RC body lift. so in total im running 7" of lift on my 2009 Chevy silverado 1500. i would like to run 35" tires with 18" wheels. i currently run 34" on 20" wheels and dont like how thin they are. im wondering about back spacing recommended to do this. im already planning on trimming some off the front and doing the "NorCal" fender mod. im new to worrying about back spacing so all knowledge and recommendations are welcomed. Thanks! pics of the lift are not up yet pics on my profile are outdated