Can you convert a 45-70 to 450ne?

A difficult question to answer without further details. Convert a trap door Springfield .45-70 to .450 NE, definitely not. Convert a No. 1 Ruger .45-70 to .450 NE, definitely. Convert a Baikel .45-70 DR to .450 NE, questionable at best.

If you do it in a No. 1 and keep the original barrel and just do a rechamber, make sure you take a pic of yourself when you touch off the first full house load.....recoil is quite "brisk".

A Baikal would handle the pressure but your shoulder would not handle the back thrust from such a light rifle. And the rifle would have a short life span as the frames on those are the small frames. Check the rim thickness carefully before you re-chamber anything. Rim thickness for a 45-70 is .070; for a 450 NE it is thinner. The only thing practical to rechamber is a .458 Win Mag, beit a DR or a Ruger #1. Anything else will be too weak or too light.

45-120 nitro. Same rim. sane brass prices