'69 Drum to Kelsey Hayes Disc - Does it change the Backspacing?

Guys, I am switching from SBP drums to SBP Kelsey Hayes discs from a 69 donor car for my 69 car. I'm ordering new wheels too and have to figure out the backspacing. I don't see why there would be a difference in distance between the hub and the wheelwell whether it's drum or disc. Has anyone had experience with that?