A 340 manifold isn't all that great.
These days you have some good intake options and that wouldn't be the deciding factor for me.
In fact, I wouldn't even consider buying a 318 unless it was really something special. If you're buying an engine just be patient and get a 360. Might cost you $200 more up front but from there on you'll be playing with 20 ft lbs more, pretty much across the rpm range, almost regardless of what mods you make from then on.