Edelbrock, Summit, Speedway Motors, Jegs, have spacers in different thicknesses and configurations.  Aluminum, plastic, and phenolic.   Joe-JDC 

JDC

Do you know the manufacturer of the 4 hole spacer in your picture?

If you plan on using the four hole spacer, then have the shop open the holes to match the spacer.  Simply install the four bolts and scribe the spacer openings.  Opening up the plenum to dual ovals requires an open spacer to scribe the outside edges.   If you don't plan on using an open spacer,  then only machine the four holes to match the spacer.  You have several options with the four holes opened up. 1/4", 1/2", 1", 2" spacers are available with four holes.   Joe-JDC

JDC

Should I ask them to enlarge the 4 openings, or create the 2 slotted holes in the bottom picture.  What gasket would I use for the bottom configuration?

Yes, you need to open up the venturi holes to match the spacer.  The lips under that spacer will cause terrible turbulence and be detrimental to your whole idea of getting the engine to run smoothly and accelerate smoothly.   That will allow the use of any of the more modern carburetors available.  If you stay with the four hole, then slotting the manifold to make a dual oval is not necessary.  However, you need to blend the bottom of the holes into the plenum to help the air/fuel turn into the ports easier.  Not a difficult task if you have a carbide burr and electric or air die grinder.  It is the little things that make any manifold work better.  Joe-JDC

JDC

Did you test-fit the carburetor by installing your phenolic spacer and then checking for any butterfly clearance issues? As long as there is no interference with the manifold, the larger bores of the spacer should have no adverse effect on performance. Only dyno-testing might confirm a slight difference, whether you use a 4-hole spacer, 2 ovals, or an open spacer, taking into account that they are all the same height/thickness.

Thank you again for your insights.

I plan to use this spacer.  Should I have a shop enlarge the holes in the manifold to match the spacer?  If the butterflies clear the spacer would it create a problem with the smaller intake openings to leave the manifold as-is?  Engine is stock rarely sees RPM's over 4,000.  I am not try to build horsepower.
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I am retired officially, but I ported heads and intakes for the last 54 years, specializing in Fords.  I always sand blast any iron intake manifold that is going to be ported before I do anything with it.  I sandblast the interior as well as the exterior and sometimes re-sandblast the interior to get a certain surface finish in the ports.  As far as the new TSP intakes, I have ported a couple of them already, and flowed several on my SF-600 flow bench.  It is a very good copy of the ECZ-B iron intake as far as flow at 189cfm, but not any better because of the small carburetor holes and small runners.  I had difficulty getting over 210 cfm with porting, and had to remove some of the ribbing at the down turns to get the flow up to 230cfm average.  I just flowed one of the newer Mummert dual plane intakes, and it averages over 283cfm.   The one I received from John last year averaged 272cfm.  You will need a spacer to fit any of the new Holley or Edelbrock or Summit carburetors unless you go with the 390cfm Holley with small butterflies.  You may have to bevel or enlarge the four holes for the spacers and carb to work properly.  Joe-JDC

JDC

I did read what you said................ From my view it's not worth the trouble of removing the manifold, finding and paying another shop to modify the manifold, reinstalling the manifold, for the problem to "mostly go away".  Then I "may" still have to deal with whatever the cut down divider may or may not cause............which I will not know until I get the car back together again.  I already ordered the TSP manifold so that's my plan for now. 

The two 57 B manifolds I found locally were rusted internally and had to get it blasted to clean up the internal passages, people selling these B manifolds want top dollar for them.  If you find a B that has internal rust find out how much it will cost to clean it and modify it.

Again, read my description of what happens with the void under the carb spacer out to the edge of the carb flange area.  Wetting, puddling, and rich mixtures.  BTDT and watched it on dyno testing.  Check out Jon Kaase's You Tube on sticking his finger in a manifold while the engine is on the dyno and the fuel wash in the ports under the carburetors.  On my tunnel ram with wrong spacers, it was soaking wet due to the reversion under the carb.  The fuel needs to be directed into the manifold plenum, and not sucked up under the spacer and wetting the gasket and puddling.  Mill the carb plenum flat and get rid of the void and use a spacer and the problem will mostly go away.  That divider cut down will never get rid of the torque dip.  Joe-JDC

JDC