Can anyone ID a 275 horse 273?

All D-Darts were built on April 17, 1966. The date cast into one of my D-Dart blocks is 2-28-66. The block P/N is 2806030-3. On the front pad below the cylinder head is hand stamped P273-4-XX. The XX on one of my engines is 26 and the other is 27.