Larry- I think what we are all saying is that just like changing carburators, when you change distributors, you need to fine tune it to your engines to get the most out of it. In many cases just bolting on the carb or distribtor will improve things, but maybe not as much as it could if it were tuned properly.
The advance specs Larry posted illustrate the point very well by showing that the original distributor produced spark on a certain advance curve which is different from the HEI distributor's stock curve. The vaccum advance is different too. The factory distribtor was set and designed for factory specs. If you've got a different cam you'll produce different amounts of vaccum (generally less with a bigger cam at WOT, wide open throttle) so you may need an adjustable vaccum advance.
Here's a brief summary of the articles I posted as a starting point for research on the topic of recurving.
The Datsun article says is that due to emission regulations the advance curve was restricted on all distributors at just about the same time the HEI came out, so all HEI's are restricted in their factory configuration, and people looking for performance out of a car that cam with HEI can improve the performance of their engine by recurving their distributor too.
Fine tuning an HEI distributor is called "recurving it" and adjusts two things: Mechanical or Centrifucal advance and Vaccum advance. To adjust the Mechanical/Centrifucal advance you play with the weights, springs and center plate until you get the right combo. A recurve kit contains several sets of springs, one set of weights and one center plate. The newer weights are lighter than the originals and the center plate limits the amount the weights can close towards the center of the distibutor. Your perfect combo may be the stock weights and lighter springs, it may be new weights and the original springs, it may be one light spring and one medium spring... The point is, you can't just swap the weights and be done with it.
Vacuum advance is adjusted using an adjustable vaccum advance cannister on the distibutor. You might check to see if you got lucky and the stock one is perfect, otherwise you'll have to buy an adjustable one, take vaccuum readings on your engine and fine tune the distibutor to get it just right.
ANOTHER OPTION for recurving the vaccum and mechanical advance on the distributors while on your engine is to use a machine/computer. I think one of the classic machines for that is the SUN machine.
