I had one post 64 m70 - a .264Win. Mag. with a ridiculous 22" bl. It was only 80fps faster than my daughter's 6.5x57IMP, which had a 26" bl.
I rebarreled it to .458 Alaskan, which I chambered up. It is the same as .450 Alaskan except the case has an extractor groove and .532" rim. The ballistics should be just slightly better than the .458 Win. Mag., due to the 7gr. greater capacity, however, Barnes data provided exactly their book speeds in my rifle. I did not feel it necessary to exceed 2,200fps with a 500gr. Hornady bullet, not the 510gr. Winchester's I found.