The Gross Vehicle Mass (GVM) is what the manufacturer spesified your van can weigh when fully loaded. Thus your calculation that you can only load 260kg is correct. The factors to recon with that carries the weight are not only the tyres and axle but the chassis of the van as well.
So if you get to a weighbridge and the weight of the van exceeds the GVM you most probably will need to pay up.
NB. It has been proven by numerous members here on the forum that went through a weight reclasification/registration excersise that the data supplied by Jurgens factory on that plate is not correct.
My advice to you is pack your van as usual and take it to a weigh bridge. You are going to get a surprise.