For your set up, you would typically need a 500 spine arrow. If you are set on cutting your 400's down, you may have issues.
You will/may be able to make the 400's work by leaving them long and adding weight up front. Set them up the way you want and then try. Only cut after you know what you are seeing is a "true" reading and not a false one.
340's are definitely too stiff and if the 400's show weak, there is something wrong!! Be very careful of false readings! stiff arrows frequently bounce of the bow (shelf or riser contact) and can give you a weak reading.
If you get a weak reading from the 400's, check out all of your avenue's...as they should be good, or ok at best but likely too stiff.
One common issue for three under shooters that will give a false reading is the shaft bouncing down and hitting the shelf. If you shoot three under, you will have a higher nock point...typically between 5/8"-3/4" and if your arrow nocks fit the string properly (tight enough to stay on the string...but loose enough to move and release easily) the arrow will dive down after release and make contact with shelf. A second nock point (typically a tied on nock) below the arrow will alleviate this issue.
You will get lots of advice, but nothing replaces proper efforts on your part. You would be surprised at how many of those "darts" are deceiving and more like lame ducks.
Another note: A properly tuned arrow will not touch the shelf/bow after the string has been loosed. If you are experiencing shelf/side plate wear, it can be time to investigate.
good luck, BigJim