[AZKen]

Here is some info from a GMC enthusiast and expert:

GMC v6 engines came with one of 3 different dizzies. First the externally adjusted style. Just like the 230-250-292 stuff. It has the spring arms to hold on the cap.
Next is the internal adjusted style. More or less the same as the later 235-261 Chevy and jimmy 248-270 302. Screw on cap.
Third is the externally adjusted dizzy with the governor control. This has 2 vacuum ports and is taller under the cap. These came on 1-1/2 ton and larger.
There seems to be no rhyme or reason for use of the first two styles. Though the externally adjusted caps seem a bit more common. Both styles have appeared on all series of 305 engines.