There is no way to predict how much a barrel will be enlarged by lapping, as condition before lapping determines how much lapping is required to make it smooth and straight.
The worst I've encountered was .003 on a 'cloudy' military barrel. A barrel that looks cloudy is actually covered with rust and tiny pits. Some barrels which are quite good will only be enlarged .0002 to .0005, but perhaps one could expect most new condition Marlins to open up closer to .001. More where tight spots are.
Never lap with commercial hard cast or gas checked bullets, OR JACKETED!
Since you intend to order a mold, and especially for a 45-70, order a bullet diameter .002 larger than groove, and sizer the same diameter. This should give you about .001 oversize bullets. For most of the relitively straightwall levergun chamberings, and including 30-30, 32 spec, I reccomend bullets which cause the cartridge neck to fill the chamber within about .002. This to guide the bullet straight on takeoff, yet loose enough for easy chambering when things get dirtied a bit.
Lap with bullets from the mold your purchase for the gun, using air cooled wheel weight alloy, or other with a hardness not over 14 bhn.
