If you wanted a 6.5x68 in an American bolt action, what bolt face would you start with, and what would be done to it?

Can't rightly start with a Magnum boltface so the only logical place to start is with a .473" ('06) bolt face.

All those I have built were on Standard Mauser actions.

If I were set on using an "American" Action I'd look at a CRF Model 70 or MRC action. I suppose it could be done on any '06 based action.

Most "magnum" bolt faces for .532" cartridge heads will hold and extract a .510" head just fine. I have a 7.5x55 (nominally .498") on a Sako with an unmodified magnum bolt face and it never hiccups.