The OEM manufacturers installed the motor/trans angled down 2-3 degrees to afford some legroom in the front passenger compartment, so the intake manifold is milled on an angle to provide level placement for the carburetor. When I do an engine swap, I install the intake manifold I'm going to use and install the motor so that the carb pad is level, then adjust the angle of the rear differential so that the trans output shaft and the pinion shaft in the diff are close to the same angle.