Your car is a Los Angeles built car (Van Nuys), so I believe your car should have the 4P on it if it were an SS 350. The CRG report states that
The 4P code was used from early production at Van Nuys, but was not added to the Norwood SS350 cars until sometime during October.
Now, just how "early" is something I don't know, so Kurt or Rich, or Daniel, etc... will have to comment.
Although the engine stamping is "correct" for a 67 SS 350/295 with a manual transmssion, and the date of August 29th lines up well to your cars build, I would ask that you post a picture of the stamp so the experts can examine it for originality.
The transmission would be a Muncie for an SS car, so is it a Muncie or a Saginaw? You might want to post a picture of the transmission stamping as well...