I assume you're a right handed shooter. Arrows impacting right would indicate a weak spine. Given your draw length, I would think the .600s would be on the weak side, though perhaps workable with a light point. However, I'd also think the .400s should be too stiff. The .500s should work. Might want to play around with more (higher) point weights with the .500s and certainly more for the .400s.
I shoot about 250-300 grains up front with Axis .500 and .400 shafts out of 45-50# bows, though I have a shorter 28-inch draw length and use a 29-inch BOP arrow.
Be sure to check your brace height and nock point to make sure they don't/haven't moved during your tuning. A too low brace height can cause the arrow to bounce off the side plate and fishtail in flight. A nock point too low can cause the arrow to bounce off the rest and porpoise in flight. A nock point that's too high will also cause it to porpoise. Sometimes a certain spine and fletching will mask these effects while another won't. That's what may be happening between the .400s and .500s. The .500s should work the best for your set up in my opinion.
One more thing to check. Axis shafts are skinny. Should work OK if your bow is cut to center, but will need to build out the side plate if it's cut past center. My recollection is that '59 are cut to, not past, center. Regardless, when the arrow is on the string, when viewed from the rear, the right side of point should line up just touching the left side of the string (or thereabouts).
Good luck.