My "Generac" 3600 rpm 2.5kw gen head is designed to be driven with a single V belt. I know this because the single pulley is integral with the fan and cannot be changed. If you were going with a larger gen head like a 5kw or larger I would certainly go with something like a double V. Many of the large 5hp electric motors in our family drying kiln used 3 and even 4 Vbelts. Worked fine.
Stan