Hey guys.....
Been lurking for a while. I'm an old fart, I worked on Datsuns back in the late 70's when they were fairly rare in NW Florida. Rebuilt a few that had been run hot, and blown head gaskets and softened the rings.
That clatter, or "cackle" as my boss used to call it is wrist pin noise. I built a 1972 ? L 16 engine, and had the same cackle at idle after we rebuilt it. You never heard it before the engine was refreshed (rings, bearings, and a hone job). One of the possible causes is if there is a cylinder ridge that was not reamed out all the way. The other one, that my old boss thought was more likely, was that with a freshly honed block, and new rings, there is more drag, and the piston "sticks" a bit at tdc, and at bdc. Any slack between the pin and piston or rod will cause a rattle, mostly at low rpm. We ran the engine for a month while we waited on a set of standard pistons, like I said, they were rare back then! New pistons and pins quieted it right down. The rattle won't hurt, I have seen engines go 100,000 miles sounding like that, in non high performance applications of course.
You have done a great job on this car, I don't think I ever saw one back when I was twisting wrenches. Mostly pickups, and B 210's. There was a Datsun dealer in Pensacola if I remember right, first one in our area.
Again, great job on the car!!!