I used 1/2" x 10" long carriage bolts with big washers on the backside of the block...The washers may have been overkill, but the threaded carriage bolts aren't! I've seen people pull on the booms pretty hard. Do it right the first time and you'll likely not have to do it over. I don't think you can get 3/8" carriage bolts 10" long, that's why I used 1/2"....