Engine Power

The 340 heads will have a weak port velocity and give a feeling of a sogooogy bottom end. The 318 heads preped would be better at this point so far. As far as bolt on and go, yea, they'll work.
If your looking for HP under 350 and have excellent street manors, the 318 are it. Ported, they'll do better than the 340 head in most lower HP levels. At 400 HP the 340 head may have good merits. Just in terms of cost assuming there ready to bolt on and go.
350 or 400 HP is a rather large gap to target for.
3.55's would be cool.