I guess I just don't understand why then is the back of the motor or block by the firewall stamped August of 1969?
I think you're looking at the casting date of the block, which lines up correctly with the suffix date of 09-15.
As for value, that's very difficult to say without seeing the car. Posting a few pictures might help, or posting a LOT of pictures on a site like
PHOTOBUCKET (or similar) would probably help even more.
The engine's not original so that will lower the value, but if you have any original documentation, that would offset the non-original engine a bit. Do you have any docs for the car?
Ed