If you are shooting an 28" arrow with 100 grain tips , I think you will be fine with the 400 spine shafts . On the other hand if you are shooting an 29 or 30" arrow with 125 or heaveyer tips i think youd be better off with the 340's . If you've got something working for you now , why mess with sucess ? , besides the drawbacks of shooting an overspined arrow are minimal , compared to shooting an underspined arrow .