OK. So, let's start with this: That's not the stock engine.
Your '69 510 should have had a L16 (1600cc). The stamped steel valve cover looks a lot like the early E and A-series engines, but it doesn;t match those either.
Looks like an early H-series (forklift) engine, based on the placement of the t-stat.
Now, what's stumping me is the Datsun valve cover (which wouldn't have been on a forklift motor) and the lack of a breather tube.
Dammit. OK, I'll be working on this for a while. Hold please.