Mareo, the pistons you had with larger pins may have been A12A pistons, they use a larger pin.
The 1600 in a roadster is different than the L16 in a 510.
It sound like your friends 510 may be a great find.
The North American '69 models had different lower frt. controll arms etc. & thus sat lower than the '70-'73 versions. They make a great handling
street/autocross car.
I'll bet you can find lots of 510 info on the web. Try this site for lots of Datsun info from the Pacific Northwest. http://www.nwde.org/
Good luck 510 hunting. I wouldn't hesitate picking it up for $200, I hate too say it, but 510's are in even greater damand than our blessed Datsun 1200's. (Though that seems
to be changing).