A serpentine belt needs more then one item to run around, like a/c, p/s, alt, water pump, etc....
I think maybe you are just looking at a ribbed, 4 or 6 grooved, flat back belt. If thats the case, hunt down a late model small block Chevy and see if you could rob the pulleys. You might have to adapt and modify, but it could be done. This type of belt shouldn't load the water pump any more then a V-belt does.
Joe
