Yes, the 450 Rigby and 450 Dakota are 458/416 Rigby Improveds.

If there were any feeding issues in a CZ 416 that was rebarelled to 450 Rigby/Dakota they would be because the bigger diameter bullet would mean the cartridge would sit a bit higher on the feed ramp and the nose of the bullet would be just a bit closer to the edge of the chamber.

In other words, if your 416 Riby with round nose bullets was feeding OK but the bullet was only just missing the edge of the chamber, then a 450 with blunt bullets might catch on the edge of the chamber.

But the "fix" would be easy as it would only require the feed ramp to be deepened a little bit.

Mike