Schlickenmeyer. Check out the 47th edition Lyman reloading manual. They have a couple of loads there. Lyman's Cast bullet loading manual has more data for several more bullets.
A case as large as the .300 Win. Mag. can be difficult to get decent accuracy with lead bullets. For the higher velocities, the bullets should be heat treated to make them as hard as possible.
To be perfectly honest, I've never loaded cast lead in a .300 magnum, usually sticking to 30-30, .308 Win. and 30-06.
Like Art said, try for too much speed and the leading will be a source of great displeasure while cleaning that barrel.
If that happens, wrap a piece of copper scouring pad, the soapless kind, (Chore Girl works fine)around a bronze brush and scrub away. Works for me.
It's all worth the effort though, once you find an accurate load. You gain more practice operating a rifle that you might not normally shoot as much, due to either cost or recoil.