What inch 400 do I need to spine the same as a 30" 340?
Hmmmm. Interesting question. You're talking dynamic spine, right? Static spine doesn't change with arrow length.
Regardless, a .400 spine equates to 78-79# static spine, whereas the 340 is about 92-94#. Assuming a commensurate difference in dynamic spine, you would have to shorten the .400 enough to achieve a 14-15# gain in dynamic spine. Was a thread a while back in which folks provide anecdotal findings for the I spine changes in 1/4-inch increments, but I can't find it and can't remember the results. If I had to take a WAG, I'd say about an inch, but that is a WAG. Perhaps someone who's done it will let us know.
Might also consider reducing point weight on the .400.
:campfire:
This would not be difficult to determine on your own, and would be more accurate than someone else's experience, since the effect of a spine change is somewhat personal to the person making the change, at least in my experience. You presumably already have a 30" .340 spine shaft. Buy a .400 shaft and cut it to 29.75", and put the same point on it you are already using. Shoot them side by side, and continue to cut in 1/4" increments, if necessary, until the bare shafts fly the same. If you seem to be getting close, you might want to change to 1/8" increments for the final adjustments.
I plugged in my numbers n the newest Stu Miller calculator using GT Traditionals. I can shoot a .400, 29", 145 tip.
To get the same dynamic spine using the .340 spine version, I had to go to 32" shaft in order to get matching dynamic spine.