Hy guys. I am the lucky owner of a rust free granny kept Datsun 610/180B.The previous owner had it kept in a roofed garage and took good care of it. In the US the 1973 610, first to appear, came out as a L18 engined car. Following year and up untill its discontinuance , the 610 came out with an L20B engine(as mine, which is a 74 L20B engined Datsun, IRS,hardtop(hatchback, I do not understand the designation "hardtop", as far as I know, there are no "soft-top" /convertible Datsun 610's(180B's)). The 6 cyl version is the longer front Datsun 810, perhaps the add you read made a mistake in writting the numbers 180 instead of 810. It could be an enthusiast installing a 6 cyl in that car too.
Check it out.
Jaime