[chris vickery]

One must remember that MTO officers can and will do as they like. Armchair lawyers take note. Better to be on the side of compliance, ignorance is not a defence, nor is "I thought..."
Trying to pass off a 1 ton plus military vehicle as a pickup when in fact it is not is only going to lead you into troubled waters.
Furthermore, it does not really matter what your registration says. Too many times the poor MTO lady at the office simply does not really know what you have either. MTO officers can order your vehicle in for an inspection and re-write of your registration if they deem it inaccurate or misleading.
At the very least you are in for a weigh in and likely a mechanical inspection as well at the scales. Remember too that MTO inspection officers are likely not familiar with the weirdness of military vehicle mechanical systems neither and can up the ante by pulling your plates and sending you off for inspection at a certified mechanic station.
Food for thought.