SHELBY’S ONLY 1967 GT500 Super Snake is set to cross the auction block at Mecum’s Kissimmee auction.

The one-off prototype was sold for $5000 back in 1967, and is expected to fetch between AUD$1.38 million and AUD$1.66 million.

When the 428ci big-block Shelby GT500 arrived in 1967, it was an instant hit, outselling its small-block stablemate – 2048 to 1175 units – in that first year.

At the time Shelby was also Goodyear's American West Coast distributor and the tyre giant asked Shelby to partner with them in a promotional event for the new Thunderbolt range of tyres.

Shelby initially decided that the GT500 would be the perfect candidate for a high-speed demonstration, but things took a turn when Shelby American Sales Manager Don McCain pitched the notion to build something truly special – and truly fast!

Shelby tasked Fred Goodell, Shelby American’s Chief Engineer, to prepare a GT500 with a special engine for the speed test, set to take place at Goodyear’s test facility near San Angelo, Texas.

Goodell took the #544 Shelby GT500 and 'rebuilt it with a special lightweight 427 racing engine; special rear axle, special transmission and, of course, Thunderbolt tyres'.

McCain spoke of the engine as, 'the mother of all 427s at that time… aluminium heads, forged crank, Le Mans rods, just basically everything inside the engine was built to run a sustained 6,000 RPM – to race at Le Mans”.

It was essentially the same powerplant utilised in the GT40 MkII, the previous year’s famed winner of Le Mans 24.

The car gained further ancillary upgrades, including: an external oil cooler, a remote oil filter, and stiffer springs and shocks on the passenger side of the vehicle to help sustain high-speed cornering.

During the test, the car reached a top speed of 274km/h (170mph), with an average speed of 229km/h (142mph) over 500 miles. The Thunderbolt tyres retained 97 per cent tread.

Shelby American intended to build 50 more 427-powered GT500 Super Snakes, but the exercise was found to be too expensive for the consumer – over twice the price of the GT500, and well beyond Shelby’s own 427 Cobra.

The car has changed hands several times, passing through a number of long-term owners. The second-last owner, Richard Ellis – collector of rare Mustangs – saw the odometer tick over just 26,000 miles with the car showing virtually no deterioration since new.

Ellis gave the car a light restoration, locating correct wires and hoses for the engine, periodcorrect Rotunda fire extinguisher, and a set of new-old-stock Shelby 10-spoke wheels with unused Thunderbolt tyres to boot.

The car presents just as it did on that day of Goodyear’s high-speed demonstration.

It still remains the only '67 GT500 Super Snake, a mythical back-room project, with the heart of a Le Mans winner, and initially destined for just one day of glory back in 1967.