Jeff, I just had a look at my RCBS cast book and it shows 1876fps for the 500gn cast bullet over 64gn 4064 from a starting load og 60gn for 1741.

I know that when I was working up the loads for my 404 Jeffery I went with slower powder than 4064 - varget . I ended up getting to over 2300fps with H4350 but it is the lead alloy compisition that will be the limiting factor. The slower the powder, the slower the start and the best result. Bulllet design comes into play along with alloy but 1900 should be easily obtaind in the 458.
A cast bullet will get more velocity than a jacketed bullet for the same powder charge so going by my Hornady book a start load for the 500gn jacketed bullet with 4064 is 66.4 gn for 1900fps and corelating that with the RCBS cast bullet book shows that you should get your 1900fps DEPENDING on the alloy and lube being up to the velocity. I just finished working up a load for a friends 458 Lott but he was only after a pig killer so didn't have to chase anything other than economy.

Von Gruff.