Something in the .670 should work fine. As for load? Lots of opinions. I've hunted turkey for decades. Most of those years with an old Savage "Fox' SBS 30" F/F and 2 3/4" "duck loads". Killed a bunch with this set up. Won a Bennelli Nova turkey gun several years back .. cammoed, handles 3 1/2" shells. I tried those shoulder busters. Short story,a 3" "turkey" load is plenty. If You want to go 3 1/2", have at it. Been there . ain't goin' back !!

Try several loads. The one that patterns best is the winner ... good luck this season.