318 Magnum

What the pistons are will give you a much better idea of what cam you can run. If they've been replaced and are anywhere close to zero deck the 274 cam would be fine.

340hp manifolds are stock manifolds. They perform slightly better than low perf manifolds but according to a dyno test I saw it wasn't much difference. That was performed on a 300 hp crate engine. If they are put on a 400 hp engine it'll be even worse cause it needs to breath better.