Technically, it's just a Ford Mark IV, not a GT40.
1. Grinding - Using the Ferrari Seasonal and Circuit de la Sarthe, you can grind $20M credits in about 2 hours.
2. The easiest way, in my opinion, is to use a '67 Ticket. Get the '67 Ticket, backup your save game to a USB flash drive, then enter the game and cash the ticket in. If you don't get the Mark IV, quit the game, overwrite your game save data with the USB file and retry.
I got 4 of the '67 cars that I wanted this way. 👍
Saved me $67M worth of grinding and I got two of the four on the first try with two of the tickets.