yes you can put a tang sight on the Ruger #1. Most the schuetzen shooters I know (who use Rugers) use a tang sight, but you need to remove the tang safety, and drill-tap for the tang sight.
I have a friend who is selling his Ruger #1 and it is already setup with a MVA mid range tang sight. It's not a .45-70, but a .32-40 Badger barrel, with a spirit level globe front sight too. Great 200 yd gun!