Ok, well I know we've chatted before on this but I wish to emphasize(sp) this on your numbers with the 15-20% factor you use.
"Approximately 550 to 735 cars": To me that seems right if you count into effect the number of states and people per state to the number of people in demand or supply on these; and let's not forget about our good neighbors up north: Canada(Canada is not MY specialty on the #'s there). I live in California and have followed these cars since 1982. I will tell you that my best guess is that 99% of every car I saw here in the past 20+ years was a 350 SBC.
Van Nuys made a ton of these in respects to the 396 offered. On Van Nuys cars in my history of viewing/finding/locating these, I have only come across about 10 actual 396 VN cars(and these I will tell you were #s cars)and mine being one of them.
Simply put, include the Norwood build #'s of these, and I'd would guess about 150(maybe closer to the 200 mark possibly) or so were assembled TOTAL, combining the two plants, with the 396 and then you have to break it down to the HP & Transmission options available. To me, any HP rating or Transmission combination coupled to a 396 originally in a Z11 is rare enough!
Finally I wish to tackle your last suggestion:"but that's backed up with some large samplings of data"....
Sampling:
The definition is as follows

Websters Dictionary)
a small part selected as a sample for inspection or analysis <ask a sampling of people which candidate they favor>
We need to ask you to define what it is that you suggest your samplings on: Total 69 Camaros produced? Convertibles produced, 396 engines actually installed at GM originally?
Lastly, you state a "Large" sampling of data: PaceCars for 1969? or.....???
My interest here is moot as I do not have actual records, but only what my trained eye and mind have been exposed to over the years seeing these cars. Yes, I do Love these cars, have had mine for over 20 years. I cant say that Ill ever see 150 of these together in one place, so how would you place your number so hi at 735 396 PaceCars?
All in do respect Kurt, I just would like to know your friendly opinions on this...Thanx!