Two observations from experience:
1) the .500 #2 (.577-.500 #2 Express) is an oddball case, not easily made from any other.....
- .500 #2 base diameter .641" rim thickness .060"
- .577 Nitro/BPE base diameter .660" rim thickness .050"
It is possible with minor lathe work to make .577 3" into proper dimensions. Pretty much need a forming die as well if you go that route.
Rim may need to be built up thickness to headspace properly.

2) I have a Cogswell & Harrison 1870's Swinburne/Mahindra/Martini type action (2xx) serial number sporting rifle in .500-.450 #1:
470 NE brass does indeed form nicely into .500-.450 #1 2 3/4", the .470 NE rim is too thin, however one can make it headspace OK on the shoulder. The other alternative is build up the rim thickness with a washer or snap ring to fit the rim recess in the chamber.
My easiest solution for my rifle was using .50 3 1/4" Sharps basic and trimming/forming......worked great and the base diameter and rim dia/thickness was perfect. (There were no SAAMI/CIP chamber specs in those days!)
- Mike