I probably wouldn't want to use a 1968 autopilot for such an arduous voyage.
I think the problem with the vane gears presently being used in the GGR is their design. Every vane gear being used has way too many moving parts. Quite simply, an outboard rudder and a trim tab vane gear would be my self steering system of choice. The parts under the water are reasonably small and would most likely survive any number of knockdowns. The only part of a trim tab vane gear that is perishable is the wind driver and the spares for that are simply some tubing and sailcloth. When I was on Joshua, I saw that Bernard used a trim tab vane gear. I don't remember him mentioning any vane gear failures in his books, and I don't believe we discussed it.
My trim tab vane gear worked from 6 to well above 60 knots, though at 60 I felt it was my job to be at the helm to assist it once the vane gear was below the wave tops, and it hadn't the wind to steer.
My wife follows the GGR most religiously, and we discuss it quite a bit. It seems (not supported by any statistics) that the majority of the knock downs and pitch poles happen when the sailors are below, perhaps sleeping. That would make sense, as my heavy weather go-to is to be on the helm sailing the boat through the storm. Obviously, that wouldn't be possible for a singlehander.