Are you using (or going to use) progressive linkage? If you are then you could run a smaller one for the rear for better milage and when you want to open it up, kick in the forward larger carb when the secondaries com open on the first carb. Tom T (telriv) has been suggesting this as a better alternative anyway.
Me personally, I am stubborn and choose to run "non-progressive" linkage on my 425 with twin Edelbrock 625 AVS Thunders. My belief is that you feed all the cyllinders equally throughout the throttle and RPM range and have 2 identical idle circuits.
Erik