Tom,
May I give you my impressions of the .45-70 cartridge? It is a real powerhouse even at BP velocities. I shoot an Uberti 1885 HighWall in .45-70 and find it to be fairly tame with the right loads. I load 28.gr of 5477 with a hardcast 405gr lead bullet and get 1300 fps which is about the original loading. The load is very accurate and the recoil is mild. I have loaded the same 405 gr bullet with 65 gr FF and 54 gr of Shockley's Gold and I can tell you it becomes a different animal. The velocities are about the same but the recoil becomes abusive. More powder, more recoil no doubt about it. I have used 300 gr, hard cast lead and have had good results and the recoil is a bit milder. If you intend to kill paper targets there is no need to beat yourself up with heavy loads and if you are planning a hunt with the Sharps then your BP velocities will get the job done.
Pedersoli sells a variety of tang sights and depending on money and intended usage you have a good selection. I would recommend the "HadleyCup" eye piece as the openings are adjustable which is very helpful.
Enjoy the Sharps.