Hi,
This link explains how to unlock your phone. Use the version for sprint since you have a sprint phone.
http://forum.ppcgeeks.com/showthread.php?t=42503
This link explains how to flash our phone.
http://forum.ppcgeeks.com/showthread.php?t=43266
As for radios, the sprint radio currently on your phone should work just fine for alltel.
However, you can flash the radio at anytime after you get your phone working with the alltel ROM. Just leave it unlocked.
Some different radios you can try are here:
http://forum.ppcgeeks.com/showthread.php?t=46616
If you run to any trouble just ask, only try to post questions to this thread instead of starting a new one. This will help the Mods keep the site cleaned up. They work hard enough for us as it is. lol
Good luck!