Crap
I used this exact wheel on mine. with 125 lbs tongue weight it stinks. even though it says 300lbs that's a lie. CRAP. Harbor freight is good for some things this is not one of them.
Cut the tube above the current wheel mount.
Weld a new flat plate to the tube and bolt on this one or a simular pnumatic version.
http://www.harborfreight.com/10-inch-pneumatic-swivel-caster-38944.html
I used this exact wheel on mine. with 125 lbs tongue weight it stinks. even though it says 300lbs that's a lie. CRAP. Harbor freight is good for some things this is not one of them.