Because that four and six seemed to appear with the 62 Nova many folks called them the Nova four and six. Remember at about the same time GM introduced The Corvair. Though it was really an outgrowth of the old Stovebolt 6 which got pressurized oiled in 53? The old stovebolt was built under license to both Adam Opel of Germany and Toyota, Gas Powering 1 ton+ Trucks from both, used during WWII. Sometime during the 1960's Toyota finally built and used a different six. They continued to use and slightly modify that old design until then. Used in Toyota Land Cruisers. It would not surprise me if those roots show in that hot Toyota 6, the 2JZ?
IF GM knew what shame was they would be ashamed that they never tried to really help that engine family along. The head re-design in about 1976 was a band-aid and for 1979 they threw good money after bad at the 250 to meet emissions. They never turbo-ed, cross flowed, OHC'd or multivalved it!
Pontiac did their OHC version re-casting the 250 block with a Poncho bellhousing pattern.(I believe the give away was they didn't change the crank flange so it used the Chevy type flywheel pattern. Earlier there were "Pontiac only" 215's, but it was identical to the Chevys and used the Chevy bellhousing pattern. About 1970 Pontiac started the Ventura II which was a badge engineered Nova and in 73 when B and O(of B,O,P) decided to have their version of the Nova, the Omega and Apollo followed. (The beginnings of "corporate engines", no longer Chevy's or Buick's ETC), many had the Chevy 6 and ending in 73 the 307 small block, 2bbl.