They are called 'shear pins' for a reason...so that they will break and not cause further damage which using a grade 8 bolt would do.
Get some new shear pins (split roller pins) to use.
The shear pin (and the bolt you used) broke because something caused excessive pressure/torque.......broken tooth on drive gear, misalignment, rusted/dirty rails.
Problem may be at drive side vs slave side.
Drive side has a driven gear that cross tube attaches to and a drive gear on other side. Either drive gear could have a missing tooth causes a slip/hesitation/momentary jam which breaks the shear pin
In order to inspect you will need to remove underbelly pan OR just cut a three sided window (leading uncut edge towards front) to gain access.