[Lynn Eades]

Darrin, The crab dizzy (21A) vacuum brake is operated via drilling back into the timing cover. The older (divers helmet- 68) dizzy has a pipe going directly off the dist housing. It would require a later timing cover as well.
The vacuum brake is important. It stabilizes the spark timing and alters timing to suit throttle position.
The early dizzy has the reputation of being the better one apart from a few downside issues, like being harder to get in and out. They are said to be more water proof (does it make a difference?) You would need the correct conduits as well because the crab has two ht leads that cross over to the opposite side (unlike the divers helmet that sends 4 leads down each side to the 4 plugs on that side)