get box reinforcements from home depot/lowes and get a 6x5/16 bolt/screw/washers for the back axle and just drill holes for the front 4 bolts on your pro-model (guessing you have a promodel)?......just bring the deck with you to the store and find the right size square tubing......it's sooo easy dude....you'll figure it out man.
ok seriously it's starting to annoy me that everyone is saying to get square tubing and metal for bars and reinforcements at home depot/lowes. They do not sell any of them there, and for what they do sell, none of them fit.