Not knowing anything about cars and stress points etc
I got the tow bar place to fit a heavy duty tow bar on the back of the wagon
then fitted heavy duty Monroe shockies
then hitched up the (heavy) Baravan caravan and used load levelling (anti sway ??) bars
not had any problems (yet) sits level, tows OK
been there for a few (10+)years now
but I wouldn't trust my judgement Tim
at the time nobody seemed to know what the wagon could tow.
I assume from looking at some of the old photos that there were no regulations in 1958
usual response was " you'll know it's too heavy when the back comes off the wagon "