Doesn't your Fleetside bed already have wheel tubs? They came that way from the factory. Is your tire clearance to the sides or top? If the sides, 1" should be plenty. You'll want more clearance at the top of the tires, though.
Your issue is the rear hump in the frame. They were pretty high until '54 (or was it '53?) when the flat top bedsides showed up and the bed floor and frame hump was lowered.
Putting a deep bed like a '59 will need to either set higher (with additional cross sills) to clear the frame hump, or move the bed floor up. Moving the floor up will likely be more difficult to do, as you'd likely want the tailgate hinges moved up so the horizontal tailgate is on the same level as the bed floor, or if you don't care about that, there would be a step up from the tailgate when extended up to the bed floor.
Good luck with your project.
