Jason, I' m assuming that the 340 is a stiffer spine (I don' t know) then the 400' s, and you have mentioned that you can not get the correct spine, and you are sort of in between.

Here is a little advice, try using the arrow length to your advantage to get the correct spine. You may want to try to lengthen the arrow by an inch or so, and maybe even go to a heavier tip. You may also want to increase the poundage of your bow. Since you are in between, I think that you made the right choice by going with the stiffer arrow. With a little tweeking, I' m sure that you can get it right on.

If you are already at the stops on your bow. Then I would suggest going with the less stiff arrow. Backing off on the bow weight a bit, keep your arrow as short as possible, and go to a lighter tip.