1958 Apache Step Side to Fleetside Conversion

[RostaRob]

I'm wondering if anybody can help me out with some info. I have a Stepside that I want to convert to a Fleetside. I already have a donor Fleetside bed and need to know if the rear axle (backing plate to backing plate) needs to be different from what was used for the Stepside? I would measure from plate to plate but I don't currently have the truck here with me....it's getting some minor rust repair. Just wondering if anyone else has done this and can share your experience. Thanks, Rob.