Putting spacers/isolators between the carburetor and the intake and/or blocking the crossover channel between the cylinder heads might help a
moment against evaporation of the fuel when the engine is turned off warm, but during
driving, there will be no difference at all in that respect.
However, these spacers can cause a variety of other problems while driving,
vacuum leaks, reduced flow through the carburetor, problems with peripherals
to be mounted on the carburetor.
The intake SHOULD thus become hot and heat the carburetor while driving in order to
avoid icing in the carburetor venturies, AND, the heat is one, of three, factors
which helps to vaporize and atomize the fuel into a combustible
gas.
The other two factors are the gas velocity through the carburetor throats (which increases
with the engine speed) as well as the amount of fuel (which must be increased correspondingly if
either of the two previous factors is reduced) which means increased
consumption.
Just use the factory style flange gasket and you´ll be fine.
