Are you sure you have your belt right, and the alternator mounted like OEM? It seems to be waaay high and wide for no reason, making your single belt pretty long. Your picture shows space for a second belt on the crank & water pump pulleys, and the purpose of that is to provide some redundancy on the cooling system drive but you'll need an idler of some type for adjustment. Like Bug says, you ought to be able to find brackets that will work pretty easy, either used or new. It's not like they only made a few SBC's.