There were a couple different manifolds with forward holes.
On a '65 Vette with A/C, there is a bracket that extends from the water pump and meets the lower hole of the manifold. The alternator is bolted to the manifold through the upper hole and the other bracket that extends from the water pump sits in front of the alternator wedging the base of the alternator between the bracket and manifold. A J-bar is used to tension the belt.
But the '65 Vette exhaust manifold has the top hole recessed from the bottom hole.
On manifolds with the holes as you show for the Impala, the engine typically uses the long water pump so the alternator sits further forward.
No Corvettes ever used the long water pump, but there may have been other year Corvettes that did find some way to use that manifold forward mount so check the assembly diagrams for the various years to see if you can find what will work. You may find that these other years that used that manifold had other brackets that used accessory bolt holes in the heads or on the intake manifold on the 350s.
I seem to remember having one wit hforward bolt holes and if I did it was for a Corvette and probably a '70. But it may have been a bit different.