before you go and install a bunch of heavy duty springs to haul around a heavy load, think about what you have for brakes to stop the heavy load.
a stop by your local spring shop seems to be the next step. if doing this for a long time daily driver you would do yourself a favor if you simply get a set made to fit your truck. a new set of spring hangers may not be a bad idea to match up with some wider springs or, for a softer ride with no load, install some overload springs above your regular springs that contact some frame "slipper" brackets when the truck is loaded sufficiently. that way you ride on 1/2 ton springs until you get a heavy load, then the overload springs help out. still gotta be able to stop the whole thing though. the 3600 may have come with this set up. you could remove a few leaves from the stock heavy set up and still have a comfortable ride if you keep the overloads. you could also install some air helper springs for those days when you haul something heavy. check here for a pic of the overload leaves and the air helper springs. available at a camper shop if you wanted to see them first hand
https://www.truckcampermagazine.com/...ft-suspension/
check here for a pic of what I mean about the overload springs.
http://chevy.oldcarmanualproject.com...esto/51128.htm