Where is it leaking? If it's just the small seal for the input steering column shaft, it's pretty easy to replace the seal with the steering gear in place.
As Xcudame says, try the teflon tape first on your existing tube. It will take up some of the "slop" in the threads. Just snug the tube up but don't go beyond the point of the pickup not being horizontal to the pan.
I know there are guys who have used that gasket/windage tray combo on stock pans with success. Moroso pans do have a flat gasket surface but not in a stock looking pan.