As per Kevin's comment above: is it better to wrap the square tube uprights in wood where they go into the stake pockets? Or just steel on steel as snug a fit as one can go? I think the wood option would risk the least damage.
IMO, Steel on steel would be better, as a wood upright in the stake pocket would eventually loosen up, There's a set of thru holes above the stop in each pocket for a bolt, and unless you put a tube thru your steel upright, tightening a bolt thru it will crush both the pocket and the tube a bit.
The issue though is getting a tight fit in the pocket, which would be easier with wood than steel.
