Say Portugal isn't a part of NATO, Would it be successful ?
Getting Portugal out of NATO would require a much more hardline and isolationist Estado Novo leadership, which would have consequences...
Also, you'd need to break Portugal's long standing alliance with Britain somehow...
Speaking in purely hypothetical turns, I think that a Spanish militairy victory would be feasible. Franco has the numbers, and most of Portugal's militairy assets are still stationed in the colonies anyway. That said, the new Portuguese government is not going to give up without a fight, and civillian resistance will be huge, both against the Spanish invasion and the restored Estado Novo.
I really can't picture any scenario in which something like this could happen and not cause the immediate colapse of Francoist Spain