The Easton 340 are going to be stiffer than the 400, that is assuming they are same length. If your new arrow s have not been cut yet, then leave them longer than the 29" that your friend has.. I would say pick your weight that you want up front,start out with arrow longer such as 30.5.. then shoot to test arrow flight, while cutting arrow off 1/4 inch at a time.. granted it may be a pain having to run back in forth to local archery store to get them cut, but it could save lot of money in the long run. Because every 1/4 you cut off a carbon, the spine gets stiffer very quickly and it can take a lot of weight up front to get it back to a softer spine.