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As you may know, simply putting two single quad application carburetors on a 2x4 manifold won't give you the performance that you desire.  IF you dig in the Forum archive, you will find several recommendations for modifying '56 series Holley 4000 carburetors to work in a 2x4 configuration:

'56 Ford / Mercury Holley 4000s originally purposed as single 4 barrel carburetors on 292 or 312 engines need to have the following modifications performed on them to function properly in an 'E' code (dual quad) configuration:

1)  Install replacement secondary vacuum diaphragm springs
- springs to be installed will have half the tension value of what were used for original single quad application use;
- this is necessary because the vacuum signal for the secondary throttle plates vacuum diaphragm is cut in half with the 2x4 set-up;

The spring value that you need can be obtained from Holley or a Holley parts supplier (Holley offers a spring assortment 'kit' that they sell for tuning the vacuum secondaries on their 4150 / 4160 carburetors;  The kit is helpful for individuals who have changed camshafts.  -Increased camshaft valve overlap will change the vacuum signal and negatively affect when the secondaries will open)

Depending on your engine combination (cubic inches, compression ratio, camshaft specs), you will likely use the white or yellow spring from the Holley assortment pack...  NOTE: the weaker the spring in the vacuum secondary chamber, the sooner the vacuum controlled carburetor secondaries will begin to open.

2) Install secondary balance tube fittings in each carburetor base after removing the existing plugs.  These fittings are installed on the fuel inlet side of the carburetor aluminum base nearest to the choke linkage side of the carburetor.  (Look at the ‘E’ set-up on your ‘Bird)  The installed tubes are connected by a vacuum hose that serves to equalize the vacuum signal to the vacuum control diaphragms to assure that the secondaries in the two carburetors open simultaneously.
 
- Ted Eaton on our Forum has recommended the use of Holley 4150 / 4160 carburetor float bowl connector tube for making the press-in tube fittings.  This tubing will look nearly identical to original 'E' carb tube fitting once installed...
- connecting the installed tube fittings by a neoprene vacuum hose will be identical to the original Ford 'E' set-up.

3) For the dual quad application, install the correct high speed primary jets in both carburetor bowls for your elevation (#45 for 0 to 15,000 ft.)

4) For the dual quad application, silver solder closed the secondary 'venturi metering tubes' (2 in each carb), re-drill orifice with the correct size number drill bit (#55 for 0 to 15,000 ft.)  Note:  obtaining the correct size secondary metering tubes is impossible;  by silver soldering the existing tubes closed and re-drilling them, you accomplish the same thing.

5) The power valves in each carburetor fuel bowl will need to have two or three turns of their spring tied together to reduce the total spring tension.  This is necessary because the vacuum signal is reduced when the two carburetors are used together in the2x4 configuration.  Without making this modification, the 'economizer' (power valve) will not open as needed to enrich the air fuel mixture when accelerating. 

I agree with what you say except for carb jetting.  The ECZ-M carbs are not dual quad carbs.  The dual carbs have restrictor rings on the boost venturii to increase the air flow in the primaries, thereby increasing vacuum at the fuel discharge tube.  55 jets will be too lean without the rings.  I would suggest leaving the original jets in the M carbs and when the setup is running, tune the jetting as needed.  I'm not sure why the secondary jets are smaller for the dual setup, the secondary throats are the same size for both carbs.  Maybe there are differences in air bleeds somewhere making the jetting requirement different.

These are still more expensive than I'd like to see you pay, BUT... I don't have any to offer to help you out.IF you want to wait, I'll keep lookiing... 

Mine will just be for crusing and to look different when I open the hood. I may need to go back to the black rocker covers and remove the T-birds but not sure on that....Once again good luck

The cover used on the 1956 EDB carbs and also on both of the 1957 "E" carbs are the same part number and again different than any of the other T Pots.As to that auction on the hold down clamps - that guy screwed me on a set of "E" style clamps (sent normal hold downs) and was a real P.I.T.A$$. I would not entertain any purchases from him.I have a Purolater dual quad Ford correct air cleaner that I would sell if interested (not cheap).I could offer the dimensions to you if you like.Oldmics

I too would like to see what the difference is.Oldmics

Car enthusiasts have been throwing multi-carb set-ups on vehicles for years-and-years without doing anything to the carburetors.  They ran, just not as well as they might.  They looked good and the operational quirks they displayed were part of the aura of having souped-up your car.  The motor heads who raced may have sorted some of the quirks out, but often they didn't do much more than jetting changes because they were only concerned about wide-open-throttle performance. 
The modifications identified for the '56 FoMoCo Holley 4000s are the 'tweaks' that will deliver a 2x4 set-up that has good throttle response, drive-ability, and the secondaries will actually open.  -Performance equal to what came off the Ford dealer lot in 1957 as an 'E' code car.  IMO, '56 FoMoCo single quad application carburetors can be modified to perform on the road just as well as the original '56 EDBs or '57 ECJs.  I see the ECJs and EDBs as a simpler carburetor than what was used on the '56 single quad applications.  The ECJs and EDBs don't have a low speed to high speed transition circuit.  As John indicated, Holley added brass rings to the primary venturi boosters on the ECJs and EDBs to work against low speed over-carburetion that would likely be termed 'flat-spots'.     
Actually, you're off to a good start with what you have.  Using two of the same series of '56 Holley 4000s (you said you had two 'Ms') you're avoiding differences in carburetor internal circuitry that none of us have a handle on.I hope you'll reconsider putting the 2x4s on the '56.