You've got it backwards, it seems. Going to .500" deflection is going to a weaker spine than 340's and would be underspined for a bow that needs a shaft that deflects.340".

An arrow that is so stiff it cannot be deflected by the spine tester would have a deflection of zero. On the other hand, an arrow that deflects a full inch in the spine tester would be very weak.

The lower the deflection number, the stiffer the spine. The hgher the deflection, the weaker the spine.

With your specs you can shoot 125 grain heads and feathers , no problem , should up your foc considerably , a guess is 13-14 % , forget the 500s , they are way too weak

I shoot a similar setup, 63lbs, 28" draw I ran my specs through thearcheryhut calculator here's the result:

cx300 28.75 inches, 8.1 grains / inch 232.875
nock 8.7 grains
insert 11 grains
3 blazers 15grains
Montec 100 grain

arrow weight 367.6
KE 64lbs
FOC 12.4%
280.fps

3 pass throughs at 14,16, and 32 yards

I just ran your numbers and came up with a FOC of 10.9% with the spine being marginally stiff.

If you go with a 125gr head the FOC will be around 13% and a slightly stiffspine which is better than a slightly weak spine.

I would not advise on cutting the shaft back as that will stiffen the spine too much.