Mr. Truman, following an uninterrupted hour of discussion, gave his enthusiastic approval. "As we left the President's office," Marx recalls, "he stopped us at the door to say, as near as I can remember it, 'Make a good picture. One that will tell the people that the decision is theirs to make. ...this is the beginning or the end!' Outside the door, we realized that we had not only what we had come for but also a title for the motion picture.   "The President had named it… 'The Beginning or the End'. "   When Marx returned to Hollywood, after further research at such atomic centers as the University of Chicago and Columbia University, and a discussion of the film's religious aspects with Cardinal Francis J. Spellman, he reported that he had found the most amazing and most human story he had ever heard. Particularly at Oak Ridge. where he had talked to scientists, wives and laborers, had he been impressed with the fact that, the atom bomb story is largely their story.   In Hollywood with the picture now definitely on the schedule, Norman Taurog, remembered for his "Boys' Town" and other outstanding pictures. won the directing nod over a host of top bracket candidates. The picture was given a No. 1 priority at M-G-M -- first call on Hollywood's most extensive list of stars and featured players, first call on the vast production resources of the world's largest studio.   Next step was the assignment of writers. Bob Considine, famed war correspondent, responsible for "Thirty Seconds Over Tokyo", was named to write the original treatment. Commander Frank Wead, U. S. Navy ( retired), was selected to do the screenplay. His latest had been the highly successful , "They Were Expendable". Considine continued the research begun by Marx. From the East, where he maintained a constant contact with the real-life players in the atomic story, he wired his material page by page to Hollywood where Wead took over.   As soon as the story outline began to reveal itself, camera crews branched out from Hollywood to all parts of the country. Quietly but earnestly, filming began on Hollywood's atomic drama -- in the Pentagon Building and the White House in Washington. D. C., at Oak Ridge, Los Alamos, Alamafordo, New Mexico, University of Chicago and Columbia University -- wherever any portion of the real-life atomic drama had been enacted.   "We'll meet again after the war"