Just ordered what I thought was the right part but isnt. What went wrong here?

 

There are a few variations that fall into basically into passenger vehicle vs truck and wide vs thin belt. The 8RT Truck engine should have a wide pulley 8RT-8509-A. The ones you apparently ordered are configured with the 78-8509 pulley which is used on the earlier flatheads (37-48 Passenger, 39-48 Mercury, 37-47 Pickups)