The 36 passenger and pickup frames were tapped for both at the factory so putting a 36 pickup on a 36 passenger frame is easily accomplished. The mounting of a 40 pickup on a 36 frame would be a bit more of a task. The frame rail dimensions are the same. The mounting holes for the front fender/headlights on the 36 would not be the same. The runningboard holes may not be the same. The blocks for mounting the cabs are not the same. The beds are not the same length, so the bed mounting holes may not be the same. The rear shock mount holes are not the same. The steering column mount is not the same. The brake pedal assemblies do not interchange from what I remember. The center crossmember in the 36 is set up for mechanical brakes.The rear bumper brackets are different, I suspect. If you are building a modified vehicle, the rails will work with tapping new holes to align everything.