The subject of 450 bushmaster and the Mosin-Nagant rifle was brought up about four and a half years ago in this bolt-action forum.
I did some calculations, and produced a very rough prototype of a 452 cartridge based on a sawed-off 7.62x54R case, and wrote some posts.
In the posts I linked to a number of web pages to show my sources. Most of the links are still active, except the one to the Russian site. (It pictured a splendid Montengrin revolver chambered in the 11.3 Montenegrin cartridge.)
Link: Had an idea for a 45 or 458 caliber Mosin Nagant
As you indicated in your post, the extractor problem is central to a good conversion. I've not tried to play with a cartridge having a rim and body smaller than the 7.62x54R. I did have a bolt face opened and the extractor modified to take a 348 Winchester-based wildcat. The 348 Win case is conisderably larger in rim and base diameters than the 7.62x54R case. However, that project ended before any shots were fired, and the rifle went down the road.
Please keep us posted on your thoughts and work.. It's almost certain that I cannot pursue this further.