Why not build a 340?

340s are nothing special once you view them as cores. Any of these engines, even the "lowly" 318, respond to the usual performance rebuild work, and all will make decent power for the OP. While technically accurrate in terms of reasoning, there is no reason to even remotely consider the main bearing size on a build such as this, or probably 90% of builds currently being done. If you're building an endurance engine, or in some cases a high rpm drag racing engine, then it's probably a worthy considertaion. For a hot rod with a 318 it's a waste of grey matter.