Re: ?? long range Lever Gun??
For LR, I would look at the 45-70. No brainer to get this to shoot way out past 500yds. Just look at the Black powder Silhouette game.
The big block with the lever actions is that you have to use a flat nose bullet if feeding from the mag. Not good for BC. If you don't mind single feeding then the better spire points now available will work well.
Just get someone to screw on the same type of barrel and chamber used in the sharps type rifles and have at it. The twist, and rifling should cater to cast bullets.
Lee has some nice looking new designs and even has one that can accept gas checks. Probably designed for the 458 Win mag but would work great in the 45-70 at higher vel and smokeless powder.
Of course, there are lots of shooters using 40cal and 38 cal so these Black powder style cartridges will also work well. Get the heaviest, pointy bullet you can.
If just plinking, then the 30 cal will also work. If also using for hunting, 40cal and bigger.