Not sure how much this will help, but...
Use any bolt that will fit, i don't think the bolt holes are threaded into the chassis.
HOWEVER... I would recommend grade 8.8 hardened steel, and the largest ones you can get through the holes.
If it is just the ball that you are bolting to the rear cross member, then any place that sells towing stuff should have them the right size and long enough to go right through the chassis, with nuts to fit.