Belt guidance

I'm putting a 360 magnum in my '67 dart, setting it up as an LA would be, OEM LA type alternator and Saginaw power steering brackets. I bought what I thought were the correct v-belts for the alternator (goes on alternator and rear balancer pulleys) and water pump/powersteering/fan (goes on water pump, power steering and front balancer pulleys). Both of these belts are about 4 inches (guesstimate) too short. Neither works for either side, but were advertised as "small block mopar" appropriate belts. Am I running them on the wrong pulleys? The pulleys I recited align properly. Does the alternator belt go to the water pump as well, maybe that's my issue, the altnerator front tab can be inside or outside of the bracket... The part numbers I have are Gates 7480 and 7370. I don't have AC.

Does anyone have part numbers/recommendation for what belts I need? If I measure using a string, can order by length?

Brian