I don't know. Surely the name referred to the cubic capacity of the engine, hence a 5000 GT would be 5 litres.
Although in the picture you can see it has a 5500 GT designation on the rear end whilst in the book the cubic capacity of the engine is described as 3856 cc, which is why it was supposedly going to be a 3800 GT; was the 5500 GT name there just there to confuse?!
It wouldn't be a 5300 GT if the engine was only 3856 cc would it?