Chains or straps are legal, as long as your combined working load limit satisfies the minimum prescribed by the FMCSR.
If a chain is not stamped with it's grade, it defaults to grade 43, the weakest grade. So if you have a 5/16 G70, you may want to put a little paint on a link that's stamped, making the stamp easier to find.
Regs on square bales. I don't know if it's been added to the book or not. :