Explanation: There are four across entries that are models of 4 x 4 vehicles. 9A, 20A, 54A, 65A. If you literally do the math and multiply 4 times 4, you get 16. The sum of 16 + 16 + 16 + 16 = 64, the meta answer. I tried to put some Easter eggs in to point you in the right direction. 25D are lyrics from “When I’m Sixty-Four”. The 33D clue has the word Commodore (for Commodore 64). 39D and 22D combine to make CHESSBOARD which has 64 squares. The 36D clue has the word Nintendo (for Nintendo 64). The 34A clue has SUV which was intended to get you to the realm of 4 x 4s. Lastly, the clue for 53D literally contains the answer in Spanish.
Fun, thank you! (OMG it was like The Usual Suspects or The Sixth Sense when in the big reveal I realized that there were so many hints all along the way!)