This is an excerpt from the Mercury Cougar GT-E registry.
While it is true that all 357 1968 Cougars built with the 427 were GT-Es, it is also true that 37 additional GT-Es were built with 428 Cobra Jet Ram Air engines. The switch to the 428 for regular production came in May, 1968 and the last known produced 427 has a scheduled production date of June, 1968. Because the GT-E was a mid-model year introduction with scheduled production dates beginning in January, 1968.
The W code 427-4V engine was found in prototype Cougars with a door tag date of 04G, or July 4, 1967. There was an overlap, where the 427 engine was still produced when the R code 428CJ engines were avaliable in 1968. After April 1968?