I was just looking at your photo, when I noticed that your alternator is mounted on the right side of the block, where as mine is mounted on the left down below the radiator hose. Was this something datsun changed during the production run? I know that the early A12's (dizzy near the front) had the alt on the left.
The alt in my car was probly moved to this side because it would hit the battery mounted the way yours is. The reason I ask is that one (both?) of my spare engines is like this and I fairly sure that is was like this from the factory.
I other thing, the A12 in my car was a replacement engine from japan and was originaly fitted in a 1200, the other two are 120y engines.

Chris.