the datsun 510 (68-74) used a rear differential made by Fuji Heavy Industries. it is called a R-160, named after the diameter of its ring gear, in this case being a 160mm ring gear. many other japanese car manufacturers used Fuji Heavy Industry rear diffs, including subaru.
the problem now is finding a subaru with a clutch-type LSD R160 (usually 3.70 to 1 ratio). the problem is that it was an
option to have the LSD unit in the cars that do have them, so i can't just tell you what year/model to go rip the diff out of. the one i just put in my car came out of an 88 subaru RX. below is a link to an article that will better explain how to find, identify, and install a subaru R160 LSD into a 510. it is really easy to do.
the stock 510 unit is an open 3.90 to 1 R-160 differential.
i recently installed a subaru clutch-type LSD R-160 (3.70 to 1) in my project car, it kicks ass!
here are some pics of the subaru LSD i put in my car to help you know what to look for. i bought mine on e-bay, could have probably gotten one alot cheaper if i could have found it in a junkyard, but i am impatient.
there is a great FAQ on subaru LSD rear diffs here-
(third button down on the left of screen, then you will have to use the mouse and grab the separation and pull it over to the right to get a full screen look at the article.)
http://kmhafer.datsun510.com/