rear disc question

[68c10owner]

My 78 C10 I picked up has a rear disc kit and Im wondering if it will effect the backspacing needed for custom wheels. Right now it has stock steelies so its not a issue but Ive heard it can change the backspacing required and plan to order custom wheels soon. Im not sure of the brand of the kit but it does have a CPP master. They actually dont seem to stop too well but I havent had a chance to do anything to it since its stored at a friends house.