The 400GT 2+2 may have looked a lot like its predecessor, but it was in fact a different body and longer wheelbase. This car had enough room for 2 people in the rear seats, while the 350GT only had room for one person or luggage, a very awkward configuration.
There is bound to be some confusion when talking about this car. The official name of the car is the Lamborghini 400GT 2+2. This is a different car from the Lamborghini 400GT, which was a very rare variant of the 350GT with a 4.0 liter engine. Only 23 of those were built.
The V12 engine in the 400GT 2+2 was much longer than the engine in the 350GT. It produced considerably more horsepower and really pushed Lamborghini forward in the exotic car market. The drivetrain was also greatly improved with the help of a Porsche style synchromesh on all gears.
Only 247 of these cars were made by Lamborghini, each one by hand. Your chances of finding one of these for sale are about the same as you getting a date with Scarlett Johannson. They do pop up on eBay once in a blue moon, so keep your eye out. You can buy up the dozens of other Lamborghinis listed on there at any given time while you wait.