Its a rimmed cartridge which presents problems with most bolt actions. Enfields and Siamese Mausers have been used for 45-70 conversions. A potentially sweeter proposition has to be a 450 Marlin in a short action bolt. Some of the older Winchester M70 short actions could handle 3.1" cartridges. Most short action bolts at least 2.88". Would make a wonderful 450 Marlin when loaded to 65,000 psi.
As has been pointed out by EsoxLucius, a rimmed cartridge is a bear to fit to a rimless action. It can be done but only by a very skilled and knowledgeable gunsmith. Another way of saying that is, for enough money, anything is possible.
Other problems with the 45-70 come into play if you have more than one 45-70, and who doesn't. I used to have a Siamese Mauser so chambered and several other rifles of lesser strength. Of course I developed a foolproof method of reloading so I could tell at a glance what level of cartridge I held in my hand. Then one day I watched as a shooting friend slipped a Siamese bomb into a trapdoor! I got my hand between the hammer and the firing pin before the boom but it scared my witless. I have since sold the Siamese and now make it a point to load only to the level of the weakest arm in my collection.
Good thing the original ballistics are pretty phenomenal. With the original ballistics, complete penetration on most game animals is a given. I can not now see the need for any improvement.
I am sure others have varying opinions and that is good. Hopefully you will get a variety and be able to make your choice.
Consider the 450 Marlin in a bolt gun. Even as a custom gun it will be easier (cheaper) for the smith and you can have the gun of your dreams.
Gibbs Rifle Company makes a nifty 45-70 bolt gun based upon a converted No4MkI enfield. Also, the Mosin Nagant can be made into a reasonable facsimile of a first model Win Hotchkiss, only it will be one shot. It definitely has a 19th century look.
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