You would do well to look at the Lyman smokeless loads for the 45-120 BEFORE you try what you suggest. Several people have blown rifles while trying smokeless loads in cases that big!

The above loads are about 5% below max for the 458 Win, from the Accurate Powder reloading manual. The 458 Lott IS a smokeless round, slightly larger than the Win, capable of holding about 85 grains of powder. XMP5744 is designed for reduced smokeless powder loads in large straight wall cases.