The first 400 GT, commonly referred to as simply the 400 GT or 400 GT Interim, was essentially the older 350 GT featuring an enlarged, 3,929 cc V12 engine, with a power output of. Twenty-three of these cars were built, with three featuring aluminium bodywork. The second 400 GT, commonly known as the 400 GT 2+2, had a different roofline, and minor sheetmetal changes compared to the 350 GT and first 400 GT. It was first presented at the 1966 Geneva Auto Show. The larger body shape enabled the +2 seating to be installed in the rear, where the 350 GT only had room for luggage or +1 seating. The bodywork was designed by Carrozzeria Touring. The 400 GT 2+2 also had a Lamborghini designed gearbox, with Porsche style synchromesh on all gears, which greatly improved the drivetrain. When leaving the factorythe 400 GT originally fitted Pirelli Cinturato 205VR15 tyres. A total of 23 units of the 400 GT Interim and 224 units of the 400 GT 2+2 were built from 1966 to 1968, when it was replaced with the Islero. The 400 GT 2+2, 400 GT Interim and the 350 GT all shared the same wheelbase.
The Lamborghini 400 GT Monza was a one-off two-seater sports car based on the 400 GT "Interim", featuring unique bodywork by the shop of Neri and Bonacini, who were already previously known for their work on the "Nembo" series of Ferraris. Giorgio Neri and Luciano Bonacini were initially hired by Ferruccio Lamborghini in 1963 to construct the chassis of and assemble the first prototype Lamborghini, the 350 GTV. Following this, they supplied some early production chassis, before turning that job over to Marchesi once series production of the 350 GT was well underway. Through this pre-existing relationship with Lamborghini, Neri and Bonacini were commissioned to create a one-off two-seater sports car based on a 350 GT chassis and a 400 GT V-12 engine. Possibly built for an unknown American client to race at the 24 Hours of Le Mans, the car was completed in 1966 and named the 400 GT Monza. The original name was the "400GT Neri and Bonacini," but the final name Monza was eventually chosen due to its brevity and evocation of racing history. The aluminum body was hand built in the Neri and Bonacini shop and went through many revisions during the fabrication process. The final result was a distinctive fastback, with a long hood and Kamm tail. The designers integrated a roll bar into the extremely thick C-pillar, possibly increasing roll-over safety but compromising rearward visibility. Other details included a very low and raked windshield, prominent but non-functional air vent grilles behind the front wheels and stylized "400 Monza" badging. Overall, the design shows visual similarities to other contemporary Italian sports cars such as the Bizzarrini 5300 GT and the Lamborghini Miura. Homologation problems prevented the car from ever racing and the American customer who commissioned it apparently never received the car. It was instead displayed at the 1967 Barcelona Motor Show on the Lamborghini importer Amato's stand. A wealthy Spaniard purchased it at the show and the 400GT Monza remained in his family after his death in the early 1990s. In 1996, auction house Brooks discovered the car in storage when they were contacted by the family of the now deceased owner to appraise some other cars. After nine years of negotiations, the car was finally sold at Bonhams' December 2005 auction in London for £177,500 GBP.
