RE#7 works well for full power loads, as does H4198. H322 is another good powder for them.

My friend uses 49.0gr. Re#7 with 400gr. Barnes. That is an over 2,000fps load in his Stainless 18 1/2" guide gun, some 150fps faster than Speer's book. It is only a 28,00psi load, according to Speer. His rifle runs it up a bit more, but it's more than safe in the Marlin and a GREAT bear load.

The 350gr., FN or RN Hornadys are really find moose/elk bullets when driven around or over 2,000fps.
The Hodgdon Annual Manual contains some excellent data for the various strength levels of the .45/70.

Using Re#7, IMR8208XBR, IMR3031, IMR4198 and H4198, you can exceed 2,100fps in the Marlin without exceeding 40,000psi, still well below the Marlin's 43,000psi max. pressure range. H4198 will almost make 2,200fps with them.

I used a 2,260fps load with a 350 Hornady from my .458 2" with good results on a big bull.