That kit is not bad for a medium duty rig. For a real beefy setup, do your own or buy the kit from Front Range Offroad (
link)
"fabricator" on the Wild Yoats BB also sell them for $40 shipped, but I'm not sure if he has any left or not.
Most people use 1/4" plate on both sides with some kind of support or sleeve for the upper bolt hole. The bottom ones will be rigid enough that you won't need to sleeve them if you use plate on both sides. I started out with the allpro kit (same as Marlin's), but decided to plate one side for more strength. I am going to plate the other side as well I have decided. Using plates instead of just sleeves will also give you a nice flat mounting surface so you don't put excess stress on the mounting ears of the box.