No, what does 6.0 have to do with it? Don't I need 6.1.3 ipsw?
OK, so correct me if I am wrong...
Device currently has 4.1 and I want to go to 6.1.3 untethered.
1. upgrade to 6.1.3 normally thru itunes
2. get the newest redsn0w
3. download 6.0 ipsw for 3gs
4. open redsn0w and point it to that ipsw
5. put 3gs in DFU mode and jailbreak with redsn0w