The Remington 405s, while a cheap bullet, are a bonded-core design, and have been very accurate out of everybody's Marlin that has tried them. I've used them on deer and hogs and both go down like they were pole-axed (honestly, I hit them so that the round went through the boiler room and
a shoulder. Beauty of the .45-70 in moderate loads with heavy bullets is it won't destroy much meat, but hits like a mortar.
Meister bullets are damn nice, as are Cast Technologies LBT series. All are available from Midway, Graf's, Cabelas, etc.
I would stick to 405-gr and heavier. You might have to pick up an oldeer reloading guide, as most of the new ones don't list data for bullets heavier than 405 grains anymore (although I believe both LBT and Meister come with some loading data with each box of bullet).
However, I REALLY like the Crater bullets from this guy Jae-Bok Young
He sells very hard, very big bullets in the LBT LWNGC (Long Wide Nose, Gas Check) design, and will email you the loading data if you don't have an older manual.
One thing to keep in mind is that almost all of the really heavy bullets exceed teh SAAMI specs for overall length. So I would make a <font color=red>*</font><font color=red>*</font><font color=red>*</font><font color=red>*</font><font color=red>*</font> round and seat it progressively in and test it in the rifle until it feeds reliably from the magazine. marlins are very good about this, and will feed Garrett's 540-gr bullets (which he tells you are seated further out than is even called for in loading manuals to hold the "firewall level" powder charge) with no problem, so it should cause you any headaches.
Speaking of firewall levels, I wouldn't load the .45-70 real hot. I've found the best velocities were 500-gr at ~ 1400fps, 405-gr at ~1500 fps, but YMMV.