Need help upgrading 3GS old bootrom 4.0 unlocked 5.11.07 BB

[MiniOzzy3]

I need to preserve baseband and keep unlocked. This above all the most important thing.

1. Do I just create a custom ipsw with 4.2.1 and redsnow 9.6 and then restore through itunes, with cydia of course. Theen unlock with Ultrasnow again?

2. I don't haave original sim, so I think I need to use SAM to activate. I am currently using it on 4.0, due to battery drain issue that it had. Previous owner Hacktivated it.

3. I've read lots but it hard to understand if I will have any issues with anything. I use GPS, and pretty much everything else on the phone. So if its going to disable any features I will stay at 4.0. But some apps I want are only running on newer firmwares.

BTW I have SHSH's 3.1.3,4.0,4.0.1,4.2.1+ so I think I'm good there if needed.

Please help....TIA

Sorry for the double posting, I'm having issues