400's are going to be way too weak. Bare shaft tune a 340 and see what you get. It would help to know what your BH gr is going to be and the total length of the arrow.
Providing the following information in case you have never bare shaft tuned before...
At 15 yards, shoot a fletched shaft. Next, shoot a bare shaft arrow (same length and BH just no fletchings). If the bare shaft and fletched shaft group together, you are spined good.
(Right handed shooter) If the bare shaft groups right of the fletched shaft, it indicates your spine is too weak, if the bare shaft groups left of the fletched shaft, it indicates you spine is too stiff.
If you find out you are too weak or stiff, you could always play around with BH weight and shaft lengths to try and get spined correct, or make a move to a different shaft spine all together. However, if you decide to move from let's say 340's to 300's, you should still bare shaft tune as you may now be too stiff and still have to adjust shaft lenght and/or BH weight.