a 69 SS396 with the clutch type fan would NOT have a spacer. The flange and the shaft on the clutch is made the correct length.
As far as the 4 blade fan, I have a few of those, but I'm not sure any of them came from a 396.
Here is what you need to do. When the fan is mounted, it should be half way in the shroud and half way out of the shroud.
In other words, looking at the fan shroud from the side or the top, you should only be able to see 1/2 of the fan. This would be the proper depth for the fan to be in and out of the shroud.
Select the right spacer to achieve this setting and you will be cool !!! (get it?)