Hoosier has all the correct info as usual.
I would add that the use of the AA and AB carbs in the 57 setup is decreeded by the transmission type installed in the car.
It calls for one AA and one AB in the cars with an automatic trans and two AB carbs in the cars using a standard trans.
That was how the "E" codes came from the factory in 1957.
By the way-nice welding and fabrication work on the pan 
Oldmics