400-500 grain hard cast bullets are winner every time. The Barnes Buster is also a fine choice, as is Hornady's 350gr flat point. Belt Mountain makes a very good and very expensive 395 gr. bullet.
I have had good success with 55-57 grains of H-4895 under 400 grain bullets in my Marlin 1895. This has been both accurate and powerful in my rifle, with velocities in the mid 1800fps. The 45-70 is not picky and will take well to most medium burning powders.
A word of caution regarding the 450 TSX in your levergun, this bullet was designed for .458 Win Mags, Lotts and the like. Spitzer bullets are not recomended for use in tubular magazines. They tend to strike the primer of cartridges in front of them in the magazine when under stiff recoil. This can have unpleasant results. I don't know if the TSX is "pointy" enough to do this in a Marlin but I would stick with flat point bullets with wide meplats.
If you want factory rounds, Randy Garrett of Garrett cartridges makes good stuff and so does Buffalo Bore. Both are spendy and both are good.
Have fun and good hunting!