Stock engine, stock heat range plugs. HP273's run n9y's and the 2 barrel engines run 14's. If you rebuild and your compression is somewhere in between then run a plug between the two...
All 273 4 barrel engines were N10Y. 68 to 72 340's were N9Y with 73 340's N12Y. The plug range has more to do with performance level than compression. I run platinum plugs now, as I feel they last longer and have a wider heat range.