Since no-one else is jumping in.... I shall make a dazzling leap out of the Noob forum.
I cannot answer your question directly, as I don't know the distance, but how I measured mine was to wrap non-stretchy string around the pulleys instead of the belt, tie it off so it's tight, cut the string & measure the length, then go to whatever shop you get your belts from & start hassling them. That method should get you within 5-10mm or so of the correct belt, but you might have to buy one either side of the length you think you need, depending on how much adjustment you have. Spending money is just another joy of engine conversions, as you will have found by now.
If the belt is crossing over, then are the pulleys in line? From your description it sounds like the belt is jumping out of the pulley over to another pulley. If this is the case, then you better fix the alignment first, or you will always have problems.