Carb...

For that 557 you want as much flow as you can possible get from a 4150 style carb. The best flow per dollar is the 850 mighty demon. They run about $500.

But as Chilly said, there are people out there who have had quality issues with Demons in the past so that's something to keep in mind. But others have had good luck.

If you're going with Holley, then per dollar a 4781 850 double pumper is hard to beat. You can find them on Ebay used or rebuilt for $150-300. An 850 holley has as big of venturis as any 4150 carb out there so don't underestimate how it can run.

True a 950 is considered to be a great all around carb if throttle response is a big concern, because they actually have smaller venturis than an 850 but a smoother enty on top. But with a 557 ci, throttle response and torque would also be in abundance with an 850 holley and it would run just as good at higher RPM.

If you want the ultimate carb for your combo though and you're not afraid to spend the bucks, go for a HP1000 Holley. They are pricey but they flow the most air.

I guess there are alot of options but it just depends on how much you want to spend.