[dsraven]

possibly the 6500 front end is bigger? dunno. check the assembly manual drawings possibly. that may give you more room for driveline.
whatever you do, dont load it up with more than the brakes (especially) and axles can handle. when building know the capacity of the axles/brakes and how much the truck will weigh empty. then add the expected load and see how that jives with the axle's rated capacity. I remember when dodge first put their diesel 4x4 on the market, their front axle was over weight capacity when guys would add a camper in the box. cops would make them remove the camper or tow the truck/camper. wouldn't allow further driving as a unit.
also, check local laws etc for when you change the truck from old style to 4x4 with different weight capacity etc. you may need some sort of inspection and then something for insurance as well because it wont be the same truck anymore.
best to do homework first