The stock carb was probably a Holley 4160 600cfm 4bbl carb. Adding dual exhausts ALONE will not need a larger carb. The 440/375 motors generally had a Carter AVS 750cfm 4bbl. carb. So, let THAT be the default "biggest carb" item.
The Holley has 1.56" throttle bores whereas the AVS has 1.69" throttle bores (4 of them the same size, as some AVS models had smaller primary sides).
Even with a dual exhaust, the 600-650cfm carb sizing will work pretty well. So no real need to go bigger than that. Some still prefer the 750, though, which can work too. Shop the sales and watch the prices over the next few months (Christmas time and all!!).
Back when the cars were new, Holley had the performance image with Carter AVS not far behind. I felt the metering on the Holleys was a bit better than the Carters, back then, BUT the Holleys had a notoriour reputation for needing yearly overhauls due to their gasket material back then. So many were swapped out for the AVS, as the AVS would not need that yearly maintenance.
In more recent times, I have bought new Holleys and they had no "yearlty maintenance" issues, but then too, the metering block and float bowl gaskets are now GLUED to the areas they physically touch. Not bad on the float bowls, but the meterinr block/plate gaskets are another story, from my own experiences. The Carter AVS has NONE of that, by observation. Plus you can alter the metering rods very easily, IF needed.
The current Edelbrock AVS2 can be a good choice, as can the Summit Racing 4bbls (built from an old Ford design), BOTH with annular discharge venturis. If you decide on a Holley 4160, then be sure to get the teflon (blue) accel pump diaphram installed so the chance of ethanol degrading the existing pump diaphram is less. Notice that the accel pump is at the bottom of the primary float bowl, for bulletproof re-filling of the pump, but IF the diaphram has a durability issue, it can drain the float bowl (slowly) onto the intake manifold and valley pan. I had a pump diaphram that got leaky on a Holley on one of my cars. Luckily, it was just a seep, so it caused no problem, but it MIGHT happen over time. Or perhaps I'm being a bit picky?
So, several choices! Watch the sales for the best pricing, too.
Enjoy!
CBODY67