Help with ideas to stiffen rear suspension

Morning All,

I have a 2004.5 Sportsman 500 HO that I need to stiffen up the rear suspension (front it just fine). I have already installed 140Lb HD springs, but its still sags in the back with minimum load. I’m a Im a big guy, 265Lbs, and when I take my 50Lb granddaughter on a ride just the extra 50 lbs makes the ATV want to go straight in turns because it seems the front is not getting enough traction. With just me, its sort of ok, but put anything on the back and it seems not good. So, whats the best alternative, spring spacers? lift kit, etc? all ideas welcome.

Thanks

Brian