I'd say the steel is all within a whisker of 1/8" - so not hard to file and a little chibbling with a sabre saw probably speeds it.
I said 13/32 square above I meant 15/32 - so a 7/16 hole and then cut and file to allow carriage bolt to seat well.
Now - if I can just determine more precisely how far to move the axle.......which is I can't really do with out deciding how far I'll extend the tongue....But I'm close to all that.