The bolt on the right will become the clamping bolt for the 1 1/2x1 1/2-inch box section, which will tie the dollies together from side to side of the vehicle. Just remember to drill a suitably sized access hole before welding the nut in place! These are 5/8-inch coarse thread bolts. The 3/8-inch hole to the left is a sight hole so I’ll be able to ensure the crossbraces are far enough inside each dolly. They’re drilled 9 inches from the inner end of each one.