Hi,
I'm with Lutz on this one - but my concern is different - unprotected aluminium in contact with steel is prone to galvanic corrosion.
Two dissimilar metals in tight contact + water = shorted out battery
And it's much worse if the water is a bit salty - like in winter.
If you are happy with the alloy spacer from a strength basis, make sure that the aluminium is very well protected (paint) from direct contact with the tow-bar, tow-ball and the through bolts.
Steel may / will go rusty but it won't corrode as quickly as an aluminium / steel pair can.
Robert