Let the gun decide.

If it were my Grandfather, I would lean towards a lighter bullet for the reduced recoil, probably wouldn't be a stout load either. I would buy several brands of 140's, a couple pounds of different powder, load some up and head for the range. Once you find a sweet load, load them up and go get Gramps.

I own a 7mm Mag that does not shoot a good group with 150 gr bullets. It is a sub MOA rifle with several 140s and 160s. Still, it is minute of deer with 150s.