There should be 2 steel shims that fit inside the head with matching holes for the bolts to the shank. They are for such a problem. From installation guide:
"OPTIONAL SHIM KIT
Use the shim provided when needed to fill any gap between the Weight Distributing Head shown below and the hitch bar
used in the particular application.
It is easier to install the shim from the top down as shown on the following page. In doing so, you will encounter one of
three scenarios:
1. Shim will go all the way down to line up with the bottom hole. If this is the case, use the entire shim in
the gap.
Note: It is conceivable that in some rare combinations, two shims could be used. In these cases,
contact Customer Service at 800-632-3290 or 800-428-7303 for a free additional shim service
kit #58207.
2. Shim will not begin to fit between the head and hitch bar. If this is the case, discard shim and continue
installation. The existing gap is acceptable
3. Shim will not go in far enough to reach bottom hole.
If this is the case, cut shim in half and use only the
top half for the top bolt. Discard bottom half of shim"