Actually, thats really interesting to see a front hub with a disk brake. Do you know which make that one is?
On the subject of your motor, a 4011@72v is a massive amount of torque. The fabricated torque arms are probably a *really* good idea in this case. With these motors, typically the bolts and washers on the axle themselves are designed to take the force and need to be as tight as possible to avoid the axle transferring punishment to the dropouts in a prying manner.
The ones that Crystalyte sends out with their kits are only adequate for moderate voltages. I would try giving
http://www.ebikes.ca/contacts.shtml These guys a call, last time I checked they have oversized/overstrength washers and nuts for the 4 series motors. At least, I think that's where power in motion gets them from. They cover more surface area on the face of your dropouts, take up more thread on the axle, and have an internal nylon section to prevent them from coming loose. I would give it a shot.