The height on the insert hole on the Curt seems to be about 12" off the ground. I'd think you'd need about a 2" drop insert installed upside down to bring the ball height up to the level of the bumper. The center of the hitch ball should be in the exact center of the mounts to relieve any tendency to flex as Mark said. This is especially important since the hitch is mounted in sheet metal and slight flexing will eventually weaken it and cause it to crack.

Something like this:

http://www.jcwhitney.com/autoparts/ItemBrowse/c-10101/s-10101/p-100000328517/mediaCode-ZX/appId-100000328517/Pr-p_CATENTRY_ID:100000328517

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250