Yes, some kind of thin solid (not a part with large hollow spaces included) spacer / adapter is required when using a Ford 4100/4100-A carburetor on an original COBRA high rise intake manifold. To work correctly, a number of the cavities in the carburetor's base must be sealed from vacuum leaks. It is important to have an appropriate base gasket under the carburetor to fit around throttle bores correctly. Many complaints users have with these carburetors develop from not as good of base flange seal as Ford intended.
Engines with PCV routed to the rear of the COBRA high rise intake.
Ford had a pretty good selection of thin engineered material carburetor spacers for 4V applications but unfortunately now most are very rare or have pass holes (four large holes in a spacer under throttle bores) too small for a 1.12 size Ford 4100/4100-A carburetor.
The Ford B9AZ-9A589-A part made of a phenolic material 0.02" thick with 1.60" pass holes is a good choice for factory COBRA intake 1965-67 and a Ford 4100-A carburetor in the 1.12 size.
The Ford B7A-9603-D part is similar but 0.50? thick with 1.50? pass holes. Any good machine shop would be capable of enlarging the four 1.50? holes to 1.60? holes.
I have seen modern aftermarket spacers that look similar to these mentioned above, but I do not know who makes them or any measurements of them.
Engines customized with PCV routed to the rear of a carburetor spacer.
Ford made several versions of spacers that fit this category also. If one wants to plumb the PCV system to go under the carburetor, a 1969-70 Boss 302 spacer is a great way to go. I install them with tube at the rear.