IN THE southern African puzzle box, two pieces need to be picked up by American diplomacy -- Angola and Namibia. But the unavoidable question is: Which one should be picked up first?

Jimmy Carter chose Namibia. He felt that improving American standing in black Africa was the priority and that the key to it was ending South African rule in Namibia on internationally acceptable terms. At that point, he figured, Namibia's SWAPO guerrillas would neither need Angolan sanctuary nor draw South African fire upon Angola, and the Cubans in Angola, there supposedly to defend against South Africa, would fade away. President Carter gave this theory a fair test. Unfortunately, it didn't work, or it didn't work in time. He left without either a Namibian or Angolan solution in hand.

Ronald Reagan, although he is still choosing, seems bound to pick Angola first. This is his natural choice, since his African priority appears to be not simply to ingratiate black Africa but to stand up to the new imperialism of the Soviet Union. It is also, in our judgment, the right choice. Even if Mr. Reagan were looking at Africa through Mr. Carter's eyes, Namibia would be a long shot. In Africa, there is much genuine feeling against the Soviet-bloc military presence in Angola, where the Cubans are not merely a shield against South Africa but also a sword the Angolan government wields against Angolan rivals. An Angolan solution may well be feasible and, if one is reached, then Namibia, everyone agrees, would fall into place.

But what kind of Angolan solution? Here Mr. Reagan is hearing an argument. Some say he should openly support Jonas Savimbi, an anti-Communist nationalist challenger to the Cuban-supported Angolan government. This is the intent of at least some of those trying now in Congress to rescind the five-year American ban on support, open or covert, for the Savimbi forces. But offering such support would make the United States, again, a miliary partner of South Africa -- the prescription for disaster. It would thrust the Angolan government, which has been reaching out to the United States, back into the Cuban-Soviet grasp. It would escalate the war and hobble the effort at political reconciliation that must be the core of any sound American policy.

In some quarters it is suspected that the administration intends to make Angola its African El Salvador, an anti-Communist demonstration project. To believe that you have to think the Reagan people are awfully stupid. Getting the Cubans out of Angola is a worthy project. The way to do it not by hooking up with a racist regime that black Africans hate in order to promote an insurgency in a country whose government wants friendship with the United States. The way to do it is to help produce a political settlement that will lead the Angolans to order the Cubans out.