Maybe a .44?? build
Im hoping to get back to that build soon. why not a 444 marlin?? you already have the mag well set up for a round that long and there is not much of a rim on the 444 marlin or just do a rimless 444 marlin on cut 30-06 brass an head space off the case mouth. it would be a easy conversion from your 308. I will have a 44 cal AK of some sort in the near furture the barrel is already profiled.

You already have a 308 mag well Id say screw the grendal if you want a 6.5, an go to a straight 260 loaded light and all you will need is a differant barrel.