You will be best served with 55 grain bullets although Sierra 63's will also work, their BC is lower. 60 grain flatbase should also be fine.
In warm weather, Sierra 69's might work but you are on the brink of instability. Forget Hornady 68's or anything heavier.
Even though you can get better velocity from a 26" tube, the additional stability confered by the faster rotational velocity is negated by the increased Reynold's number due to the increased velocity.
