Will,
I suggest that the .45-70 is quite versatile already. It can be shot bone-crunching heavy, or tin-can plinker mild. Brass is readily available and reasonably priced. There is a wide selection of bullets, both cast and jacketed. Especially, the cast ones are great fun, economical, and in some designs and alloys can shoot thru most biological entities on this planet.
I would suggest shooting it a bit with the available load spectrum, before cutting steel. I like 'exotic' or 'quixotic' calibers as well as anyone. However, I have found that many of those virtues and rewards looked for in off the beaten road cartridges were already extant in everyday stuff, off the shelf.
That's my own anti rifle-loonie thesis, if you will.;~`)
and, yes, last I checked, No 1 butts fit No 3's. Much more comfortable to shoot, too. Whup some hottter commecial loadings from Buffalo Bore thru that carbine stocked firearm, and you'll note the diff, right off. In fact, in .45-70 loads, Ireckon you can get all the Amusement park shoulder ride that anyone would want -- LOL ;~`)
Happy shooting, Amigo Will.
John