I drive a 200 W P/M alternator off a jackshaft situated in the dynamo drive location and utilize a 120J6 polyV belt (12.0 inch long J section (3//32" pitch spacing??) and 6 ribs - although 4 would likely have done it but I like long belt life). I do not run it all that tight and believe that on say snapping the throttle open and shut, the belts slips some and therefore doesn't overload the sprocket/chain roller interface (OMG I've spent too much time being corporate!) acting as a torque limiter. This belt runs a speed up drive with pulleys of about 1.9 inch dia driving one about 1.4 inch dia. I was worried about belt life, always pack a spare or two, and having run this since 1993 and never having had to change a belt on the road, am starting to feel some confidence in it, I have had 3 failures during that time, twice a wire has fallen off the regulator and once the idiot light circuit (yes I know - I need one) that tells me that it is charging - failed. The unit still charged just fine and later when the ammeter internals melted (yes sitting out in 45 deg weather in Australia) jamming the needle, I had to check with an independent meter, and yes it was still charging. I replaced the reg and kept the other one as a spare, although being from a small John Deere tractor it is available pretty much anywhere in the rural world.
All this just to say that poly V belts (IMHO and having spent 20 years selling industrial power transmission stuff) work very well. I would take one over a regular v belt any day. smaller pulleys can be used, and thy give long life.
Robert