i would pick the boss 429 as it has more overall potential than the other engines listed. the only real advantage the 427 would have is if you got the sohc engine. the boss 429 was never fully developed as a race engine, and there is still more hp locked away in that motor for those who want to take the time to look for it.
I'd rather stay away from the biggies, but if that was the only choice I'd go for the cobra jet, I've always liked the sound of that name.
I remember reading a couple of years ago about the cobra jet and super cobra jet, and I read that the SCJ is weaker or thinner or something than the CJ. Is that true?