Mount the two brackets nice and tight....measure three times for the 2x2...cut 2x2 a hair long....grind 2x2 until snug enough to stay up under the truck without being welded....tacked it in place....pull the whole thing down and finish welding...had to weld in small sections so i didn't warp it.
Also used a cheap angle finder from Home Depot ($10) to make sure the 2x2 was level horizontally and perpendicular so when i add the reciever it comes perfectly straight out the back of the truck