I'd go for the 3600. You can pretty much do the same things to both, but the longbed is so much sweeter. The only bad thing is the driveline. You can't get the traditional upgrades like roller bearings and disc brakes for a 3600 like you would for a 3100. The rearend with the 3600 is another issue. Whatever gearset you have, that's pretty much what you got to stick with. Maybe a 3600 truck with the 3100 front suspension and a later model 6 lug rearend out back, you would have the best of both worlds.
