Per the Valiant.org website, the 273 was designed for the Valiant. It went into production in December of 1963. Your Nov block could have gone into a Dec or Jan produced vehicle. The block should have an assembly date stamped on it.
The assembly date is February 26. But for a power pack engine! I was thinking that it might be one of the test mules with 273 x 4 bbl. Like a Charger 273 or Commando 273!
