For passenger vans the best bet is low-profile tires (45-50%) on
17-18" wheels. These tires should be rated for heavy cars eg Merc
S-Class, which is as heavy as a loaded T3.

In fact a loaded Westy should be no heavier than a loaded passenger
or commercial van (the GVW is the same for all), so S-Class tires
should suit any T3. These tires will also give far better handling
and braking. Cost is the only consideration (other than taking more
care not to pinch the tires in rough going).

Dsespite what most listees vill swear til they're red in the face,
vans do NOT need truck tires.

I took the front truck tires off my 4WD Hiace diesel vamn because
they were worn, and put 15" tires from a Jag on in their place. These
showed up the unsafety of the truck tires... now the thing is
frightening to drive in the wet because the rear end lets go so
easily in corners!