Good luck with your rear build! I just got through doing the same thing.
The Monroe shocks are actually dampeners more than shocks so they won't help support your cart but maybe they'll work for you with those springs you linked. Have you looked into ATV coilovers instead? I'm currently using Sportsman 700 coilovers on the back of mine and they ride good, especially with weight on the rear seat but are still a little stiff with only me driving and no passengers. Still miles better than the HD leafs that were on there before though.
I just bought some Fox shocks that should be here next week that have 125lbs springs on them along with helper springs to soften the ride a little and still allow adjustment for extra loaded weight if need be.
