David Hoskin has a an excellent page on the GT-40. Here's a excerpt:

From:
http://www.ozemail.com.au/~dhosking/ford/gt40/index.html

"The GT40 was Ford's answer to Ferrari's domination of sports car racing. In 1963 the project to build a car that would beat Ferrari at his own game was begun. Just three years later in 1966 the GT40 began a 4 year winning streak at Le Mans.

The original GT40 was the Mark 1, powered by the 289ci Ford V8 engine. Although in it's first year of racing, in 1965 it wasn't hugely succesful it paved the way for the Mark 2 which dominated the racing for the next year. Later after rule changes the Mark 1s returned to win the Le Mans 24hour race in 1968 and 1969, now powered by 351ci V8s

While the Mark 2 was built purely for racing, the Mark 3 was built for the road, but wasn't much slower than the Mark 1 on which it was based. The Mark 2 was totally different, it 427 cubic inches of raw power meant in was easily able to dominate sports prototype racing and the Le Mans 24 hour race in 1966.

The Mark 4 was the ultimate and best GT40. Built on a totally different frame and bodywork, this car not only has the power (427ci) but also lightness and aerodynamics. Clocked at 354km/h (220mph) on Mulsanne Straight at Le Mans on it's winning run in 1967 it was the fastest of the GT40s."

http://www.holmanmoody.com/