I'd go 7", and certainly no slower than 8".
I have a 6mm Rem on a Long action M 700.... 24 inch PacNor Stainless barrel, heavy magnum contour... one in 7 twist....
heaviest bullet it sees is the 115 gr Bergers.... 46 grs of RL 19, 3250 fps...
the lightest bullet used... H414, 75 gr Hornady HP, 3675 Fps....
its the most accurate rifle I own.