68-70 Manifolds on 72 340?

You will be fine. I run a set with more engine than you listed and it has W2 heads on it. I got tired of fussing with headers. Got manifolds and opened the ports so they where as close as I could get them. I can not tell a difference at all. Did I leave a few HP's behind? Sure but I can not tell on the street at all. Runs great and no more BS header problems. You'll be fine... so YES they will be bolt on to your heads with no problem at all.