I beleive there is a revolver out there chambered for 45/70, called something like "BFR". Still I don't think 45/70 is considered a pistol cartridge. Call a gun shop and ask if you need a handgun permit to purchase.

For what you are wanting, I can't think of a better gun than a Marlin Guide Gun. If the 45/70 is a problem, get it in 450 Marlin. Essentially the same cartridge, but with a belted case. I have not seen a handgun chambered in 450 Marlin.