I had the same dilemma and really wanted a #1 45-70 but had to have the heavy bbl. I settled on a #1 in 458 and haven't looked back. Cast bullets and light loads of SR-4759 makes a "belted" 45-70 that shoots great and is easy on the shoulder. I know it doesn't have the heritage or the mystique of a "real" 45-70, but I sure enjoy mine.
PS at around $500 or so on the used market, you just might find a guy with a #1 458 and a box of shells with one shell shot anxious to rid himself of a beast...
Phil 4:13 I can do all things through Christ who strengtheneth me.