I just acquired this manifold, school me what happens if I use this on a stock 57 312auto.
That's what the factory used on the 270 horse '57 312s. they will probably require some amount of effort to get them to run correctly.
thinking about this manifold with a couple more modern carbs, Holly or Edelbrock's
You appear to be missing a lot of stuff that would be almost impossible to find if trying to be correct, and the carbs appear tampered with also. The manifold is also for old square based carbs, also hard to come by. If serious about dual 4s a later aftermarket manifold and carbs would be a better choice. Nice wallhanger thou or for someone restoring an Ecode if carb are correct numbers.
The front carb has '55 style secondary fuel tubes,, the rear has '56 style. Also, the secondary throttles have been modified to become mechanical instead of vacuum. Causes terrible driveabulity issues. A lot of missing items to make them driveable. Not an impossible project, but a difficult one.
I just redid the gaskets on my Holley/Ford 94's tri-power carbs. Depends on how much time you want to devote to it. You might print this out, and send a letter to Vintage Speed in Florida (yes US Mail). Ask them to call you, and have a conversatation with them.