I shoot a Bowtech Captain (dual cam) at 62-63lbs, and found 400's to be weak-spined for my bow. I replaced them with 340's and life was much better. I have them cut around 28-28.5" with a 29" draw(also using a loop).
One suggestion- don't get set on keeping those limbs maxed out. Be willing to change your poundage, as bows will often shoot better at other settings. My bowtech is a 60-70lb rating, maxes at a measured 65-66lbs in reality, and shoots well about right in the middle on the limb bolts. Lower is ok, higher is doable, but I have it set where it gave the best results.
I paper-tuned my FMJ's to bullet holes. Then I tuned the bow so bare shafts hit with fletched out to 30 yards. Then I broadhead tuned with a fixed blade (muzzy) till broadheads hit with target points to 40 or 50 (forget where I stopped, but usually by 50 the broadheads start showing a little more drop). Sounds like more work than it really was, as each tune gave it just a little more precise tuning, rather than restarting each time. The paper tune got things pretty close. The bare-shaft tune refined it, and the broadhead tuned completed it.
I love my FMJ's. I've killed two deer with them, and neither seemed to offer substantial resistance when the FMJ/Muzzy arrived on target!
My suggestion- stick with the 340's.
The way I determined that 400's were weak spined in my setup is that I backed out the limbs as far as I felt comfortable doing, and the tune issues I was having diminished greatly. I got the 340's and started putting poundage back on the bow, and presto! They worked like a charm.
At least with my bow, I've found on Easton and Victory shafts that I need to go one spine stiffer than the charts suggest.