Just looked this up in the Mopar Engine book. It says in '67 the 273 and 318 shared the same forged crank. Starting in '68 all the way through '89 the 318 got a cast crank, even the 4-bbl motors.
Also, the 340 had forged from '68 to '71. But for '72 and '73, it also got a cast crank. So maybe this is what you were thinking about Adam.