If it were me, I would find the next higher diameter
SAE bolts that you can get in Grade 8, and drill & tap
on my driveway. You would want to take some care
with tapping (1/2 turn forward, full turn back, clear
the hole completely every couple of turns etc.) but
you can drill nodular iron with a decent cobalt or TiN
coated bit with no problem. Do not break the tap off
though, or life will suck.