Kurt,
As most have responded - not a good idea to shoot 45-70 rounds in a 45-120 chamber, for you or the gun.
As an aside - You don't give a make/model/brand of the 45-120 you located or its asking price. If it is a Shiloh and you can get it for less than a grand AND it is in good shape, get it and rebarrel it for the 45-70 and sell the 45-120 barrel. If it is an NEF or IAB or etc, and can get it for $400 or thereabouts, get it and shoot it as is. If it is a Pedersoli and is in the $700 range, then it's a toss up (IMHO) as to get it, shoot it as is or rebarrel it. If you are uncertain what to do at this stage of the process, probably the prudent thing to do would be to pass on it and look for just what you want. BTW I have 3 45-70's and have no need for anything bigger (110 or 120) except for "cool factor" but would consider a 45-90. Nope, I like the 70 . . .