This is one reason we decided to go with a wood chassis, with a 3/4" thick piece of plywood on either side of the wheels. The axle bolts can be countersunk, leaving a nice flat 5.5" tall, 26" long mounting surface for the side bumpers.
The kit frame is ok...but it leads to these fun problems....
How about adding a structural piece of 3/8" plywood along each side of the kit frame, with holes to make room for the bolt heads, and countersunk mounting holes on the outside to make room for flat head bolts that attach the wood to the frame.
Eight feet of 3" wide 3/8" thick plywood weighs about 2 pounds, that would be enough to provide a handy mounting surface for the bumpers around two thirds of the BUMPER PERIMETER.
Just an idea...use it, shoot it down, whatever you want
