I don't know much about an SD33T, but I bet the Nissan Patrol forums would be a good place to start.
http://www.patrol4x4.com/forum/
These Nissan Patrol trucks were off my radar, since I always thought Nissan hit the US in the 1980's. I ran across a Patrol in a farmer's field a few years back. I was shocked to learn it was a 1968 or 1969 (forget which now). Anyways, that is what I would do for such a challenge. You'll probably find it will be more popular outside the US. If you are lucky, Canada but Europe and Australia might be the best place to source a head since you've seemed to have struck out through the usual resources. Good Luck!