If the driveshaft is lower in front at rest then when driving and you go over a dip the compression of the rear springs will pull the ds partly out of the tranny. I would think normal position of it would have it so that it is near level at rest with the driver and a full tank of gas, but a little . If it is low at the front the action of the suspension should be checked very carefully.
Other than that I'd put the engine as low as possible and as far back as possible without preventing basic service. I would not have the
oil pan hanging below the frame.