I found that several people have expressed an interest in obtaining a bolt action rifle in 45-70 caliber, most commonly done by converting a Siamese Mauser or P-14 Enfield. Many have suggested

instead getting a 458 Winchester magnum and loading it down, or using the 458 X 2″ wildcat. Now the 450 Marlin is available, and any bolt action with a H&H magnum bolt face can ve easily adopted to it.

An interesting discussion @ rec.guns.