i would not go any bigger then the 600.. i have a 600 on my 5.0 and its perfect...and with no choke you will have to let it warm up for a few minutes or it will spit and sputter when you gas it

 

I didnt go with the 600. Upon advice from others the 670 street avenger was the best for my set up. I did also purchase a cam that portmaster recommended and i have to so what an improvement. I do also have the choke on the carb