If you are thinking about a stockish build I would pay attention to piston weight. In stock form 340s have extremely heavy pistons. So if pistons need replaced and you shop around compare piston weights. Maybe machine shop could help with piston selection too.
For stock like 340 builds I recommend take a look through this article:
1970 340 High Performance Mill Dyno - Mopar Muscle Magazine
TTI makes 2 1/2 mandrel bent exhaust that will hook right to the stock 340 manifolds. The TTI head pipes are full 2 1/2 inch and no torsion bar pinch to choke flow. It will sound and run better with headers but for mild builds stock 340 manifolds not a bad choice especially with the TTI exhaust.
Also note stock mopar A Body exhaust tips have like 1 1/2 inch inner diameter where the tips goes into the outlet of the tail pipe. Crazy restriction that can be avoided with stock appearing tips from TTI and others which slip over top of tail pipe outlet so no "choke" effect.
