Really need information like bullet diameter, hardness, and a lot more, but...I tried plated bullets once, .38 wadcutters, and saw no point in trying them again. However, I would treat plated rifle bullets like cast and seat the bullet to where the bullet is slightly engraved by the lands when the cartridge is chambered. Seating short of the lands almost guarantees a deterioration in accuracy.

You could probably decrease your powder charge significantly; check a load manual with cast bullet data. The Lee book will probably work but the Lyman is better. In my opinion, Reloder 7, 5744, or one of the 4198s would be better powders, but do more work with your 3031 since you already have it. It may prove quite satisfactory.

Before you do any crimping for magazine use, shoot the bullet without any crimp. Adjust your seating die just to where the bell is taken out of the case neck and the cartridge chambers without difficulty. If you later choose to crimp for magazine use, crimp just enough to keep the bullet from moving under recoil, no more. Your loads will probably shoot more accurately without a crimp.

As for bullet diameter, most .30-30s should shoot okay with a .309" bullet, but will probably be more accurate with a .310" or .311", though you may have to turn case necks with the latter. I have no idea what diameters are offered by commercial casters. Good luck-