I am quite happy with a 350# spring on my heavy 4 seat cage.. I used QA1 14-350 coil over springs.. Mike at Prestige has some nice springs as well so I would call him as they test this stuff all the time on different bikes..
I just put a set of the 350 pound springs on the rear of my Prowler 1000 and a set of 15" ITP wheels with BF Goodrich Mud Terrain A/T tires. I now have 12.5 inches of clearance and it is great. I am now going to have UTV Crap install heavy duty tierods and frame support rods.