IF the rest of the 340 is stock, the stock manifold is about as good as it gets. Even if it's just had a couple of mods, like a mild cam and headers, the stock manifold is hard to beat.
That’s exactly what I am told it has, a mild cam, and yes it does have headers.
I think I will leave it alone for now and if I need to get into refreshing the motor then I will go one way or the other.