[JesseK]

Thanks for the responses! The CAD will clear some things up, but the idea is to attach the 2 box tubings along their 1" faces. The bolt will not go through the entire thing; instead only the two touching faces will be bolted -- like what Jim described in his last sentence. The bolt itself wouldn't be longer than 1/2".

The countersink is for chain clearance so the chain (or belt) doesn't rub against the bolt head if it jerks while slightly loose. The sprocket & chain will be housed within the 2x1. Other solutions are low-head machine bolts and/or simply putting teflon over the bolt head. Or I could put the chain elsewhere, but the new location is part of the potential improvement.