Box locks are easier than side locks as projects as a rule. First rule of projects it to find a gun with good solid barrels. Then do as much yourself as possible. Expert outsourcing different parts of a project gun becomes very expensive in the real world. Far cheaper to buy a high condition gun than recreate one in a project. Most projects are just a gun needing to be fixed up to become a shooter. Much above that, is very expensive in my personal experience. Half the fun is in the looking and half is in the work making the gun into a decent shooter.
If your are looking for a special gun, based on some memory of childhood or a whim go for it. A project is a chance to do something out of the ordinary. Far easier to buy a factory gun at Dicks than fix up a interesting gun that has fallen on hard times.
