The spacer does not take up for the "slack" because of the governor. The governor is about 4 times thicker than the spacer. The spacer does indeed insulate the carb, but it also provides those vacuum ports. GMC's may or may not have had one. I don't know. But, I do know that GMC's usually ran a Zenith carb which may have not had the vacuum ports in the base, thereby not needing the ports cut into the spacer. As was said earlier, if GM put it there, it was needed. Its not that expensive, I'd go ahead and get a new one and install it.
Buckaroo01-
The Carter YF is a direct replacement for the Rochester B/BC. When I installed mine, I didn't have to tweak the throttle linkage or the fuel line. Direct bolt-on.
