340. Even with 400 up front, I think the 300 would be stiff. I'd be really tempted to buy 2 of the 400s too & just try 'em.
I know my setup's a little different, but to give you an idea, I'm drawing 63# on a Horne Carbon longbow (not quite cut to centre) arrows are 31" (I draw 30) shooting 300gn up front & I need to use the 400 spine Axis/MFX for perfect flight.
I would imagine, carbon being as length sensitive as it is, that the extra 100gn up front would more than offset the 2" length difference.