I had a Ruger No. 1 I rebarreled to 6.5 Grendel a few years ago. I found the best powder to be VitN530 using either Sierra 123 MK or Hornady 123 AMAX.

I used Lapua cases, CCI450 primers and Murom SMR.

Best accuracy with that barrel was around 2750fps, jumping 15thou. Barrel length was 26in