I have sized the 457121 Lyman and the LEE 457405 for years with a mallet ...
Now the bullets from this "sizing" shoot actually pretty well with little problem. If you use this method... just make sure you do it base first and use a rubber mallet.
I don't get the question of the .504 die bit, but then its still early here...
You can always have stillwell tooling open up your dies for you for about $17. I do have several dies in 45 caliber including .452 and .451 and .450. I think the .450 and .451 almost useless for great accuracy but are just the ticket for the range and you still will have 4" at 100 yards off the bench. .452, .454 and .457 all work best for accuracy for me in a .451-2 bore.
Sabots with lead in my 50's (and I don't do this practice for other than grins) were always the most accurate with a larger diameter bullet but nowadays you can actually get some pretty nice sabots that have thicker walls that take up the "windage" in the bore nicely.
Aloha...
