...here are the Physics of a Spacer. On a Carbureted, 'Naturally Aspirated' Intake.
When you increase the Length of the Fuel/Air 'Column', with-in a Port, the 'Charge' MUST Flow Faster, per Time. This 'May' add to the 'Volumetric Efficiency', That Will Increase Horsepower. BUT, The FARTHER the Carb 'Butterflies' Are FROM the Intake VALVE, the More 'Lazy' the 'Throttle Response'! Because the Vacuum 'Signal', Sensing, is 'Delayed' by that Increased Distance.
...The Very Reason, Smaller Ports (2V) create a More 'Crisp' Throttle Response than Larger Ports...such as found on the Cleveland 4V Head. The Difference in Response time may not be felt/detectable...only the Dyno and a Timer will know for sure.
...'Fuel Injection' over-comes this Phenomenon, because the Fuel is Delivered 'Separate' from, Independent of, the Air Flow...Almost Instantaneously! When the 'Shot/Charge' is Injected to the Back of the Intake Valve Or Directly into the Combustion Chamber, Making the Throttle Response nothing Short of Amazing! Plus, Much Higher RPM is Obtainable.
MJ