I had the same issue on my 69, but I believe it was the reverse situation.

From what I remember, '69 was the big crossover year for water pumps. Not only did they move from 8 bolt to 11 bolt, but they also changed lengths on the shafts.

I went back to O'Reilly Auto Parts where I bought the original replacement water pump and I had them look it up in the system again.

....

I just went to O'Reillys website and looked up a 69 Firebird.

A1 Remanufactured Water Pump

Part # Price Desc
------ ------ --------------------
58-302 $24.99 with 3 31/32" HUB HT
58-303 $19.99 with 4 15/32" HUB HT

.....

There ya go. Hope that helps