The M5R1 was actually installed in 4.0s from the factory. Just not Aerostars. I believe that the reason for the lack in the aerostar was because most 5 speeds were sold in cargo vans and due to gas mileage and the fact that most cargo vans only carry light loads they didnt see the need to make a 4.0 cargo van with a 5 speed. Rangers,bronco IIs and explorers did come with the 4.0 and 5 speed M5R1. Check out this link and the application guide and the parts descriptions. http://www.drivetrain.com/mazdaM5R1.html The 4.0 trannys used heavier duty parts, which I will be putting in my 3.0s tranny.