[Big Bore]

I don't mean to be a sad-sack, but if you .458 is anything like mine, you may not be able to shoot lead in yours.  Marshall spent hours with me trying to make mine shoot.  We shot every bullet weight from 300 to 500, and diameter from .458 to .461.  The only thing I did not try was seating out longer than the magazine as making it a single shot did not appeal to me.  My .458 is a Ruger M77 and after two years of trying to make it shoot lead I finally conceded the defeat and now shoot only jacketed.  With jacketed bullets it is a sub MOA rifle; with lead, as with your gun, all bullets fly sideways even at 25 yards.  Forget about even hitting the target at 100 yards.  
 Hopefully you will get lucky and find a combination that works, but for myself, I've given up.  I spent so much money on trying to find a "cheaper" lead bullet it would shoot that I could have bought enough jacketed bullets to keep me shooting for years.  Not to mention all the powder, primers, cases, and the waste of perfectly good cuss words trying to make it shoot.

 

[Big Bore]

My .458 set, RCBS, came with an "M" type die as all straight walled cases should.  If your set does not have an expanding die, you should get one.  If your set has an expander die, and I'm sure it does, you do not need the "M" die.  
  May's catalog from Midway still lists the Lee Factory crimp die for the .458 Winny, so maybe they have them back in stock.  
 Hope all is well soon.  We'll be here when you get back.