After an increasingly successful two year program, Ford and their partner Shelby American continued to campaign the GT40 in prototype endurance racing. The 1965 version of the GT40 proved itself capable and was primary opposition for Ferrari's P2. For the new season, Ford concentrated on developing the Mark Two GT40 which would compete against Ferrari's radically different 330 P3.
The early 427 GT40
During the 1965 LeMans, Ford received much attention for two large-displacement GT40s entered in the race. These two cars were prepared by Kar Kraft, a subsidiary of Ford run by Roy Lunn. They chopped up the GT40 chassis to accept the 427 CID Galaxie engine. Unfortunately, development time on these cars was short and the decision to run them at Le Mans was unexpected.
The initial performance of the 427 GT40 was promising. The car could reach 210 mph down the Mulsanne straight and qualified almost ten seconds faster than any Ferrari. During the opening hours of the race, prototype GT40s were in the lead. Unfortunately transmission problems arose, retiring both cars. Afterwards, it was clear that the 427 GT40 would be the car to race and a more robust transmission was necessary to make the distance.
The 1965 car left many problems which Kar Kraft could not have reversed during their short development time. Although the car was basically the same as the Mark I it had several hundred additional pounds of reinforcing and a longer nose which interfered with aerodynamics. Even though these problems arose, it was decided that the relatively cheap, NASCAR-proven 427 would power the 1966 Mark Two GT40.
source : supercars.net