If you go with a skinny carbon like beman cenetershot (9/32) or the axis (17/64), I think you would be ok with the 500's.
With a 5/16" shaft like most all of the other companies, I would recommend a 600 spine. the Khunter you have should have a dacron string and that will require a slightly less spine. Plus with the 500's you will have to leave them full length or near. The 600's can be cut down if weak.
Think about this... if you get a shaft that is too stiff, there's nothing you can do to make it shoot good. Oh yes, you can load the front up a bunch but then it will shoot with a rainbow trajectory.
If your shaft is a little week, you can cut a little and if need be build out the sight plate.
BigJim