Re: .45-70 long range
Your best bet is to look at sites related to BP silhouette (BCPR). These guys know all there is to know about getting a big cast bullet down range.
My research has shown that well cast bullets ( 1:20 tin:lead) can shoot around 2MOA at long range using BP. Using smokeless like IMR 4759 should give you similar performance. Some use H870 or other 50cal slow powders to simulate the full case loads of BP - very effective. If you are going for faster then 1200fps, you will have to consider a "tougher" lead - wheel weights are a great choice, cheap too.
Bullets are easy. Any of those suitable for BP silh will work - check out Lyman and the Postell molds. Lee makes an interesting gas check design that I am considering too.
Gas checks are going to be needed as wadding is difficult to use in smokeless loads. Too much space in the case. Consider cream of wheat or shotgun buffer if this is the route you want to go-be careful.
Vel could go as high as 1800fps with smokeless. Just make sure that it is suitable for our action.
Lube will range from beeswax to Thompson blue. You will have to match that to your rifle, vel and accuracy. The Lee Alox seems very interesting. SPG lubes are always a good bet and they will be able to provide more insight given your needs.
How far can you shoot with these bullets? Historically, the 45 familily of BP cases have shot well over 1000yds. Refer to cases like the 45-110 and 45-120 (Quigley stuff). With smokeless and strong actions, you can get similar performance from the 45-70 and smokeless powder.
The trajectory is rainbow at best but there is no denying where the bullet lands. Great alternative to our high tech missle launchers. Looking at building one myself. Will be a nice challenge to try and get this "antique" technology into the MOA range.
I would expect the affect on game to be impressive. If you can hit them of course.
PS At long range, you may want to lay the target almost flat on the ground. That way you will get nice round holes... :-)