Intake/carb should be level. At least I have always been told to make it as level as possible. I hope this helps.. will
Just remember that the height you mount the motor in the chassis and the angle it takes to level the carb at ride height directly affects your rearend pinion/driveline angle...double check that before you make the motor mounts permanent...
this is a very good article on this topic http://www.streetrodderweb.com/tech/0203sr_drive_shaft_angles/index.html