I have a chance to buy a 450 Marlin at a good price. It seems that the dimensions of the 450 Marlin are close enough to 45\70 or 458 mag that these dies could be used to reload for the 450. I have both die sets. Has anyone tried this?

You'll need to purchase dies for the .450 Marlin. The .45-70 & .458 Mag. are different inthat the belt on the .450 is wider than the .458 Mag & is 1/2" shorter in case length. the .45-70 is a rimmed cartridge and I wouldn't even attempt to run a .450 marlin case in a .45-70 die.