Frank'll get it right out to you and it will be good. I bought my 20 ga. barrel from him, and of course had to put it on its own frame and stock. It's a lovely little 22 incher......perfect for around here.
I started with one frame and add-on barrels, and soon quit. First, it's a pain in the butt to keep moving them around and it really effects accuracy. I'm much happier with each barrel on its own frame and stock, tuned to where I want them. I can see swapping them around with shotguns, but not rifles. I don't think two inches off with a shotgun matters that much.
NOW THE .45-70

You have entered sacred ground. Der ain't nuttin like it. Get it on it's own frame. DO NOT THINK OF IT AS A HUNDRED YARD RIFLE. Shoot it and learn it. If a flat shootin' rifle drops four inches at 300 yards, and the .45-70 drops 48 (exaggerated), you still have to learn to work with them. It's just as easy to compensate for 16 inch drop as it is a 6. Learn the .45-70, and it will take down whatever you can see......and you'll have a special satisfaction in doing it.
For inspiration, read Paul A Mathews "Forty Years With The .45-70".
Pete