I think a good 1.5-6 or 2-7 would be a good scope for a 45-70, in fact that's what I have on mine! I would want a good field of view for quick aquisition of a hog at relatively close range, especiallly considering you only have one shot on hand. The Mueller 2-7x32 Mult-Shot would be an excellent scope, I think, and with the illuminated dot, excellent for low light shooting. If that sounds good to ya, give Jon a call at The Optic Zone for a great price.
For ammo, I'd shoot 405gr factory ammo from Remington or the 350 gr PMC, they all shoot good and you wouldn't have to worry about working up a load tween now and then. The PMC load is stout at ~2100fps but shoots 1-1.5" at 100yds and 2.5-3" at 200yds, with a zero 3" high at 100yds, it hits 5" low at 200yds.