This will get you started.....www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/How-to-find-the-balance-points-for-a-Landyacht/

For rough off track stuff I would go for a 'stiffish' mast as upright as you can get it, with a fullish sail.

The best 'off road yacht' I have ever seen was the Faze5 yachts designed by Bill Finch. there is a couple of pics in this article.
www.seabreeze.com.au/forums/Land-Yacht-Sailing/Events-Competitions/Beach-Blast/

Balloon tyres, Scaffold tube frame, Scaff mast.... it could sail over almost anything.

The evolution of land yachts over the years has gone toward low slung high performance machines. In the old days it was make them sail over anything and as bomb proof as possible.

If you are going for a jib make sure it has a roller furling system, at times you need to get rid of a flogging sail....and quick.