Greetings, Treebeard!
I recommend that you get ahold of the book, "Loading Cartridges for the Original .45-70 Springfield Rifle and Carbine" by J.S. and Pat Wolf; therein is described the best techniques for duplicating the Frankford Arsenal Cartridges with modern components. Personally, you seem to be doing rather good with the original sights, and I wouldn't change anything, except replace broken or missing parts with original, period replacements.
With proper BP cartridges, you might be surprised with what your rifle can do! I believe that you're right about your bullet lube; you definitely want to change that to a good BP lube. SPG is great, and Wolf's book gives a recipe for BP Bullet Lube that I can personally recommend as well.
Keep Us Posted!
M.T.M.