i really doubt that the frame would bend from using 3/4ton springs, if that frame bends theres bigger issues to worry about.
id upgrade to 3/4ton springs and axles if your happy with the truck as it is. there cheap and work great. id put helper bags on your stock rear springs that way you still have a decent unladed ride. as for the front, just throw in a set of diesel coils and theres lift and no sag. (i have diesel coils if your intrested)