Keep the same size bolts you have now, switch to grade 8 or if your metric an ISO 10.9 bolt. Drill a hole in the head of each bolt and use lockwire to secure them all in place. Secure the bolts in two groups of three, and make sure the wires are in the right direction to take a load if the bolt backs off. The bolts should not come loose, hopefully that will solve the issue. If the bolts continue to break even when lockwired together then there is something else amiss.

[Linked Image] Lockwire procedure

08' Jeep Wrangler
64' Plymouth Sport Fury 383
58' Triumph TR-3A