RCBS sizers do fit in my 450...however...
I bought an RCBS sizer for my .41 magnum, and the bullets kept shearing off assymetrically. This was in a wadcutter bullet, so it was really noticable.
I thought it was the Lyman mold, so I went to e-bay and picked up a used .41 sizer from Lyman...remember, a Lyman sizing die...and the bullets are perfect. So it was the RCBS sizer that was bad. Also, the newer sizers have this stupid groove in them that doesn't feed the lube as well, and you have to heat the lube up really well or the upper grooves don't fill. I use soft lube and the older designs with the multiple holes are much more efficient.
I know that RCBS has a "no questions asked" policy for returns, but I simply haven't gotten around to doing this. Besides, they will send me another die, and now I don't need one.
Good luck
rooster