Hello,
All depends on the 'build', if it's a high riser 'type' motor with a '2000' rpm power band then the 600's are the way to go. If you're looking to be able to pull away from a stop light at less than 2200 rpm without every cop in town following you, and your build is more mild then go with the 450's.
Either way if the guy building your motor say's 450's, then it's quite likely you should trust him as he knows the motor build, cam shaft selection, rear end and transmission gear's. These cars are mighty fast and dangerious with 'only' 375 stock pony's. 575 HP is quite nice and if it's in a wide power band and you're street racing then a second gear start from a slow stop light roll out will be quite fun
. Either way there's a 1 in 10000 chance of anyone pulling up to you and 'winning' with either carb selection.
Cheers,
Mike