340's spine are right at the start, or bottom end, of your draw length and bow poundage, depending on your point weight you could also shoot 400's OK but 340's should be absolutely fine. Arrow charts are a guide, there are no absolutes. Sometimes it takes some experimenting. My hunting out fit is IBO 325, 68 pound,29 inch draw and I shoot 340's with 125 grain heads. I find then a little stiff but I like the weight of the arrow so I run them at 31 inches to soften them up.
My target bow is IBO 350, 62 pound, 29 inch draw and I shoot 500's with 100 grain heads, arrows are 27.5 inches long. Not much between the poundages of these two bows but a heap of distance between arrow spine. So as I said nothing is absolute.
In your case I would try a different vane first up. I use Blazers myself but they are a fairly high profile vane. (They have to be to stablize broadheads with something so short.) A lower profile may alleviate your problems.
Still a Scout.
I'm not trying to be difficult. It just comes naturally.