Whether or not the shooting would have been justified is irrelevant IMO. The OP should have locked his doors, and should have bugged out as soon as the BGs moved to block him... but in the end he proved himself the better man. Everyone went home with their lives, if not their dignity intact. Every mistake has a lesson hidden inside it... avoiding those sorts of situations is one such lesson.
Remember that not everyone has the option of legally carrying a gun. Road rage happens plenty often, if not more, in states where CCW is forbidden or restricted to the social elite. If this happened to me tomorrow, all I'd have is a can of pepper spray. Getting out of a bad situation is always better than merely getting through it... because sometimes fighting isn't an option at all.
I remember the encounter that 'enhanced' my awareness... I was leaving a friend's house late at night in Chester, PA- unarmed, as I was returning to NJ that night. A man approached my window at a stop light (between cars but not really boxed in), and I really didn't know what do do. Turns out all he wanted was bus money- I dropped some change into his hand out the top of the window, which was barely cracked open. I didn't know he was there until he was literally right next to my car. That's when I started locking my doors, and that's about when I started carrying a can of pepper spray in the car (in addition to the one on my person, which is the best I can do in NJ). I'm also one hell of a lot more aware when I'm stopped, for any reason, especially at night.
I'm just lucky as hell that the guy who gave me that wake-up call was just some poor guy who needed change for a ride- if he'd been a carjacker or some other lesser life form, I'd have been totally screwed. I thought I was playing it pretty safe, but here some dude who wasn't even trying to be sneaky caught me by surprise so badly that it didn't even occur to me to punch the gas and run. This was a few months after I got my FL CCW license. The only reason I wasn't carrying is because I was heading for home, in NJ; I'm legal to carry throughout PA. I honestly don't know if I'd have drawn my weapon if I'd had it on me.
Sometimes the shock of realizing that yes, this is happening to you is enough to cause hesitation. Realize what that hesitation could cost you, and try not to make the same mistakes twice. If that means drawing down on a threat, then do so... but also realize that you might not always be able to do that. You might not always have your CCW with you; you might not be legally able to carry any weapon at all. Sometimes running is not just an option, it's the only viable option.
So, speaking as someone who couldn't have drawn and fired in the same situation in my own home state (or even in neighboring states if I'm only there briefly), the OP made a good call. The worst mistake he made was waiting too long to make it, but in all likelihood he saved himself a trip to the hospital- or worse.