Taking a 1990 model being just shy of 1000kg fully loaded so say 500kg per tyre, this equates to a tyre with a Load Index of 84 or preferably slightly above.
A safe bet then, commonly and cheaply available would be 165x13C 6ply commercial tyre with a Load Index of 91.
Tyre pressure would depend on actual weight of your van, suitable tyre pressure chart here, www.arcsystems.biz/Pictures/tp.jpg
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