You simply dont. The Harpoon is a class 7 car, while the Morello is a class 9 car. They are meant to be apart in terms of performance and with both maxed out, there is no way that the Harpoon can or will ever be able to beat the Morello.
Too bad, it would be great if you could jump to higher class by paying cash. Like from class 7 to 8 for 1 million, from 8 to 9 for 2 million or something like that. This way you could drive your favorite car.