A 340 with only 125gn up front would be way, way too stiff. A 400 could be about right, but even then you may need a heavy (75gn+) insert. Try a 500 spine with that point weight & cut it until it's either right or too short.
Just to give you an apples/oranges comparison, I'm shooting a Morrison Cheyenne, 63# @ 31" & to get a 340 to shoot right I'm shooting a 250gn point with 100gn insert. This gives me a good arrow weight & pretty high FOC too. If I want a lighter arrow (Handy for those long 3D shoots) I shoot a 400 with aluminum insert (13gn I think) & 125gn points.
One thing to remember with carbons (well, a few things actually...) Length will affect the spine a lot more sverely than changing point weight. A ¼" cut off will drastically alter the spine, adding or subtracting even 50gn will make little difference. Let the arrow tell you what point weight to use, don't be too set in stone on using one particular weight, it may not work out as you'd wish. Don't fuss over arrow length, you'll need to start at full length & with your draw length & weight, probably won't need to trim much, if anything. If you cut them at 32" before you begin tuning, chances are they'll be too short & you'll need to load the hell outta the front end to bring them back (length being more influential than weight...)
Most people I know are shooting arrows that are wa too stiff (Not that you can tell them any different....) For some reason people think that their bows need an arrow that is far too stiff.
Personally if I were you, I'd be starting with a 500 spine shaft & 125 point & trim & trim until it flies well. I bet it's closer to being right than you'd think
Make sure to bareshaft (if that's what you're doing) right out to 20yds plus. Again, a lot of flks seem to bareshaft to 10 yards & think that's good enough. The further, the better. I personally don't stop fiddling until I'm at 40 yds, but I'm a little more anal about these things than most.