In addition to the two holes
on top of the hitch
coupler you can also use
two on each side.
I'm thinking the top two
will be enough, mainly because I couldn't quite figure out the best way
to line up where to drill the hole on the tongue.
One thought I had was to use
a nail on the coupler side and hit it with a hammer to make an indent
where the hole needs to be drilled, but it's a difficult space to get a
hammer in and I was ready for it to be done.
Put the coupling in the U-beam and mark the end with a pen.
Then put the U-beam on a flat surface and lay the coupler along side it, lining the end up with the marker. Now you can mark the holes in one side. Repeat for the other side.