Assuming your cartridge is the standard .45-70, you can rest assured that your twist rate will stabilize 405, 500, and even 550 grain lead bullets at your stated velocities. It may overstabilize 300 grain bullets. The US Military standard twist rate of 1-in-22 inches was adopted by most sporting arms manufacturers since the middle 1870's and has been standard since. Custom barrel makers offer faster twist rates for the long-distance BPCR shooters.
Lubrication can be critical in cast bullet rifles, especially when using long barrels. Most shooters recommend that you look for the presence of a "lube star" at the muzzle after firing. A "lube star" forms when the bullet has adequate lubricate to make it out the muzzle, and the excess blows out to collect on the muzzle as little grease ridges.
