I thought that it looked familiar so went up in the loft and looked around. This is the piece that you need. You can have it for $25 including the shipping in the Continental US.
You can see how it wedges the alternator between the manifold and the bracket at the upper hole while the lower hole bolts into the manifold to stabilize it. The other hole goes to the water pump bolt.
Actually, the way you currently have it, you've moved the alternator ~3/8" forward (thickness of the bracket) and outwards a bit from the stock arrangement because the bottom hole of the alternator should be in the upper manifold bolt hole. Hope this helps.