Good day Blocker
Parts information by Volvo is by and large electronic (Known as Prosis) however dimensions of drive belts will be excluded only part numbers and illustrations, while its possible for a dealer to post an individual page I don't think this will help you so much, suggest you could try a major belt supplier often they have a data base with the dimensions. Another possibility is a Duetz dealer the "B" engine is sourced from that plant and could have a belt to match.Today most manufactures sell there products through dealers gone are those days when you dealt direct with the maker, I was fortunate to work for a outfit that was direct customer contact it had great advantage for both parties, first hand information and a honest approach to being helpful.
Food for thought
Kind regards
Uffex