Even the stock 9.00-20s will support a maxed out M35 when singled. Any increase in tire size and weight rating is simply a matter of personal choice. The original M34 trucks had single 11.00-20 tires with a wheel well in the bed, flipped hubs and rims with an inch or so less offset to widen the track.
There are pluses and minuses to both single and dual rears.
Among the pluses for singles: Buy 7 tires and rims, not 10. Tighter turning radius. Typically better cross country performance, particularly in reduced traction conditions and when lightly loaded. Unless you go to monster tires, singles will weigh less than duals and burn up less HP in getting them rolling. They can look really cool.
Pluses for duals: Lower cargo deck height. Less urgency in dealing with rear flats (singles=stop and fix a flat NOW, duals= limp to a safe level spot) In really rough terrain, you've got 5 spares. Less downshifting on hills. Full suspension articulation (tall tires bottom out in some conditions)
Lots of other things to think about but I'd start with asking myself "What am I REALLY going to do with this truck?" Sometimes when this subject comes up I'm reminded of the guy who owns a canoe and kayak shop who says "I'm always surprised by the number of people who come in to shop for a two person canoe who state that they don't have a regular paddling partner!"
If you only want the truck to look good for you or to carry light loads, do what you want. If you intend to carry really heavy loads in broken county, you'll likely want the full suspension travel and clearance of dual 9.00-20s. The final answer depends on the response to the first question and your budget.
Good luck
Lance
