Mid year 273 in 64 Darts?

According to MMC Detroit (MMC Detroit What date code should be on my part? ©), engine blocks for common motors would typically have been cast 30-90 days before the block was machined and the engine assembled. (For low production engines like 440+6 or Hemi, the time between casting and assembly would often have been longer.)

So yeah, the block of a December 1963 built 273 could easily have been cast maybe as early as September 1963.