I need Carburetor info?
500cfm won't buy you anything unless the rest of the engine has the appropriate modifications. Why do you need a bigger carb?
For that car, keep it stock, with the original equipment. If you're looking for power, this is not the way to go.
Rebuild your original carb.
And going to a 200ci won't buy you much and you will be disapointed with the performance after all the hard work you put in and it won't be "screaming". BTW, early mustangs never came with V6--no such thing.