Harness & wagon fitting queries

[DownUnder Gal]

I'm off to the hardware suppliers this afternoon to source some more bits and pieces for the shaft assembly. I want to change the back end, where it attaches to the wagon, from curved to square. Does it matter if the width of the shaft assembly at the back is a little wider than the wagon itself? i.e. if the piece of the assembly running parallel to the front panel of the wagon is a little wider than that panel? I'm going to try to get it to match the width but it might depend on the parts I can get.