I have done 2 rebarrels of 300 win mags to 6.5 rem mag. I have also previously owned 2 of the 264 win mag.
Go with the 6.5 rem mag. Both of the ones I have done feed perfectly and you will have plenty of room to set those long 6.5 bullets way out.
I am getting 3200 fps right now with 130 gr Scirrocco II's with RL17 and have run 140 gr Sierra GK's and Accubonds at 3100 fps. The 6.5 rem mag is easy to load for. IME not so on the 264 win mag
You should get much better barrel life with the 6.5 rem mag since you can use faster powders in the 4350 range. The win mag will need the slowest powder you can find in the 870 range