[blucar]

From my point of view I find it a little difficult to answer your question, mainly because you did not state what the reason for using a carb spacer on your engine was.
I assume that when you say "carb spacer" you mean a composition "Phenolic" spacer in lieu of metal.
Phenolic spacers use to be very common, thirty plus years ago between the carb and the manifold, the purpose being to minimize fuel boiling in the carb float bowl.
Since most vehicles built since 1987-88 have some type of fuel injection, the problem of fuel boiling in the carb bowl has been eliminated.
Now to your question: I think the spacer you use should match the carb and intake, some are open plenum, some are ported. Some four barrel manifolds use an adapter for the style of carb that is going to be use.. An AFB style of carb is usually an open plenum, where as a quadra-jet or Holly is usually ported, large and small. I might be a little off on this configuration, so bear with me.
I recently had a very unusual experience with a carb spacer. I have a '39 convertible that has a 2001 SBC Vortec, that I used a carb on in lieu of fuel injection. The trans is a 700R4. The build on the car was "finished" (are they really ever) in 2008. The car is a Resto-Mod, a pleasure to drive, except for two things. The car is hard to restart when it is warmed up to normal operating temp, 195. The engine cranks and cranks like it is not getting fuel. Let it set for awhile and it would start right up.
The second problem was very hard shifting of the 700R4 trans. It was near impossible to drive the car without chirping the tires shifting between 2nd, 3rd. I would try my best to soft peddle the accelerator because the chirp of the tires would make heads turn, plus it was very uncomfortable with such a hard shift.
I had several mechanics look at the carb and check the trans.. The car is an Edlebrock AFB, not by my choice, that was the choice of the builder. The major problem with the trans turned out to be the guy that built the "first 700R4". He used a 3,800 RPM stall speed torque converter and a vette valve body. The shop that built the trans had given us a racing trans.. They went out of business before we got the car running. I had to get a new trans from a different shop..
About two months ago the car stalled, would not fire a shot, had the car hauled to a shop that I recently started to trade with. The module in the HEI ignition had died. I had specified a GM HEI, the shop that built the car used a GM knock-off from China.. The distributor was replaced. I mentioned to the tune up man the hard starting, he "SAID, the carb needed a phenolic spacer" the fuel in the carb was boiling. He called the local Hot Rod shop, they had one for the AFB, Edelbrock makes then. I went and got it. "Bill" the mechanic installed the spacer, the car ran fine, I drove away..
I could not believe how well the car drove, acceleration was smooth, the trans shifted smooth as silk.
I returned to the tuneup shop, told Bill what was happening, he got a smile on his face. He thought the carb spacer was a no brainer, was surprised that no one else had figured that out.. He had experienced the trans shifting problem before, the problem being the differences in manifold heights compared to a stock GM manifold with the GM trans control cable and carb bracket. The added height of the spacer solved the shifting problem.
I know this is a long story to a short question, It cost me over $4,000. and five years, only to have the problem solved for less than $25.00.