To keep recoil pleasant and impact mean, get an LFN 320 to 350 gr and keep velocities at around 1400 to 1500 fps. If you run them faster than about 1600 fps, they won't kill as quick, but if you want, to flatten the trajectory, the bullet will stand more speed than your gun can, or you can enjoy.
I have no clue what size the meplat is on your RCBS mold, but calculate the DV it delivers at various speeds and you may find it suitable for game if you cut the speed back.
Throat length doesn't matter, as you have way too much powder space anyhow. Diameter should be large enough to make the loaded rounds a close fit to the chamber, so the bullet gets started straight. .460 is good for most 45 rifles, but some take a bit more and some with tight chambers a bit less. Groove diameter isn't important when fitting the mold. Bullets will size down as they go in, and be accurate if they are sent in nice and straight.
I recommend no crimp groove for all 45 rifles as the throats vary a lot. With your single shot, no crimp is needed, but if you use the bullet in a magizine gun, like the Marlin, seat out till they touch the rifling lightly and crimp there with a Lee Factory crimp die. Crimp lightly. Just enough so bullets don't get driven back into the case while banging through the magizine tube.
