Vice President Bush yesterday said that leftist revolutionaries governing Nicaragua will find the United States a better friend than Cuba's Fidel Castro if they "reject the totalitarian course" they are on that will lead their country to "madness and ruin."

In a Miami speech made available by his office here and which Bush said was directed at the Nicaraguan leadership, he said that if the Sandinista government joined "the community of the free Americas . . . you will have our help and our friendship. It is not too late."

Bush did not say what Washington alternatives the Sandinistas might face if they do not turn away from Castro.

Informed U.S. government sources said that among options being studied by the administration is a Latin and South American naval force that would blockade or interdict suspicious shipping headed for the Nicaraguan coast in an effort to stop the flow of arms from Cuba.

Those arms, Washington charges, go to leftist guerrillas fighting the U.S.-backed civilian-military government in neighboring El Salvador.

Such a blockade effort, they said, is still in its infancy and would probably take until late spring or early summer, if ever, to evolve. The idea, the sources said, is to try to find "a Latin solution to a Latin problem" that would, if possible, leave U.S. naval forces out of such a blockade by using proxy forces.

The plan is one of several options as the administration searches for ways to stop the arms flow. While Secretary of State Alexander M. Haig Jr. has consistently declined to rule out publicly any U.S. actions against Cuba or Nicaragua, several highly placed officials tend to play down speculation that some direct U.S. military action may be imminent.

From the start of the Reagan administration, Haig has been the official warning repeatedly in public about communist dangers he sees in Central America and about the possible need to "go to the source" of the trouble, meaning Cuba.

In recent weeks, Bush also has become a frequent spokesman on the subject, seeming to play the role of offering friendship while Haig continues the hard line.

High-level U.S. sources said that while there is some administration disagreement about the effectiveness and political wisdom of potential military moves in Central America, the administration is united on the notion of the nature of the threat in the region and the seriousness with which communist takeovers would be viewed here.

"There is enough evidence," as one official put it, "to believe that Nicaragua is rapidly becoming a communist country right in the heart of Central America. If that happens," the official said, "it will be another base similar to Cuba" from which to launch "the overthrow of other governments in El Savador, Costa Rica, Honduras and Guatemala."

These sources suggested that President Reagan has not decided what, if anything, he will do in the region.

One official said the president "is very sensitive and does not want to appear to be the big colossus from the north in the eyes of the people from the south. On the other hand, you can feel sure that he is not going to stand around and do nothing."

If the need is clear, the official said, "I don't think he'd shy away from taking action."