>>I bought my Carat with nearly-new tires on it. They are Sigma Shadow
>>95T 205/70R14 with a max pressure of 44 psi. I can find a load rating (apart
>>from 1521 lbs at max pressure) on the tire but I'm asuuming they're C rated.

Mordo,
Isn't the "95T" the load and speed rating for your tires.  Check the Vanagon.com tire information page, I think "95" is close to the recommended number.  (some of the tires on that list show a load index of 95 even though the number at the top of the page is 97 or 99 for a syncro)
Unless you overload your van there is no way you will weigh more than 6084 lbs. (4 x1521 lbs.)  Keep your total weight below 5000 lb. and you will have more than a 10% safety margin.
George
'85 Westfalia