"...an action wrench..." And a barrel vise with the proper bushing. Getting the barrel off is no big deal. Finding the .45-70 barrel likely will be. If your smithy has to make a barrel from a .45 blank, you're looking at big money. Machining time is expensive. Then you get to muck about with the mag to get the lips right.
The existing barrel any good? Good or decent No. 4 barrels are difficult to find. If you go with the .45-70, you'll be easily able to sell the .303 barrel to help off-set the cost.