I take it your ramp is dropping off at a 6 degree angle. So it drops off about 4 feet over its 40 foot length.... in other words at the rate of about 1:10. So assuming your kingpin is 40 inches in front of your tailgate you shouldn't have clearance issues so long as you have over 4" of clearance above the tailgate on level ground.
If the ramp is going up at 6 degrees rather than down and the centre of the axles is 16 feet ahead of the rear of the trailer the ramp will rise 1.6 inches over that 16 feet.... so you won't have to worry about your leveling jacks dragging.
