I know a couple of people have already said it but it's worth repeating... Absolutely do not use a threaded bolt in shear like that! The entire weight of the front of the car is on them, not to mention the cornering loads which can more than triple the force applied to that bolt. Threads cut into a bolt are like perforations in bathroom tissue, it WILL eventually break. A good airframe bolt has a shank to span the required length with 'rolled' threads at the end for clamping. Rolled threads are better than cut threads as they don't damage the hardening of the bolt. I would definitely use an airframe or ARP bolt ( or at least a Grade8, or 10.9 metric)with a long shank and collars of varying lengths to adjust the Castor. Also, that bracket looks to me like it would flex all over the place during cornering. Maybe weld some ends on it between the two verticals like a box, or gussets on the outsides of them. Just my two cents, I applaud the lateral thinking though!