with what little measuring i have done over the years, it seems the centerline of the crankshaft through the transmission output shaft ends up roughly 3* down when the [carb] mounting pad of the intake manifold is level.
however, engines have been mounted in all kinds of planes over the years, [relative to the vehicle's chassis] so who knows ? shruggy
in operation, the vehicle goes through many different motions, so the carb flange may, or may not, stay level.
toss everything out the window with fuel injection.
just my feeble mind at work here. your mileage will vary.
beer