PwnageTool Wont Make Custom 4.0.1 ipsw for 3gs

Hello!

I have an iPhone 3gs running 4.0.1 with baseband 5.13.04 and old bootrom (I think its the old bootrom;
the S/N starts with 88930).

The last few months my 3gs has been acting flaky and running down the battery very quickly. I replaced the
battery with a new one from ifixit.com but its still acting the same way, so I would like to try and reload 4.0.1
but PwnageTool isnt letting me create a custom 4.0.1 ipsw.

I've tried Pwnagetool 4.01, 4.13 Unlock Edition and 4.3 and ALL of them say "Wrong firmware bundle selected"
when I pick 3gs, Expert and select the 4.0.1 ipsw. I verified the SHA1 hash matches whats on theiphonewiki.com
Archive, so what am I missing?

This in on Snow Leopard v10.6.8 and iTunes v10.5.3

Bonus question: Do I have any options for upgrading my 3gs to anything higher than 4.0.1 or 4.0.2 WITHOUT
upgrading the baseband to the ipad 6.15.00 version?

Thanks!

Oh, I almost forgot. I need to use the Ultrasn0w carrier unlock, as I believe this 3gs was locked to AT&T, but I live
overseas and travel to many countries, so I need the carrier unlock.

Thanks!