Something worth noting is the manifold appears to have a vacuum log cast inside for power brakes or PCV etc.
Here is one, not sure if same as yours above Steve,:
https://www.ebay.com.au/itm/Ford-351...wAAOSwUn9aCsFB
I have had other Speedmaster stuff before, not as good as Edelbrock, Holley etc, as the one above needs some work to remove the excess flash etc, and like all manifolds it will need to be angle matched to your heads, and then port matched. At least this one says made for 50mm throttle bodies.
Not sure what would happen if opened up to 58mm with those tiny vacuum ports near the runners.
Gary