“Late Night With”… Conan O’Brien?NBC is going down to the wire with its choice for a host to replace David Letterman, with O’Brien — a producer on “The Simpsons” and former writer for “Saturday Night Live”– among the leading contenders, according to sources. O’Brien is reportedly the top choice of his former boss, “SNL” exec producer Lorne Michaels, who will produce the new “Late Night” show. NBC exex, however, are said to be still holding out hope of landing a better-known marquee talent in negotiations expected to continue through the weekend. A one-time head of the Harvard Lampoon, O’Brien began as a performer with improv troupe The Groundlings before becoming a writer on “SNL” and subsequently “The Simpsons.” NBC ran a test with him earlier this week on “The Tonight Show” set for studio officials, agents and assorted writers, where he did a brief monologue and interviewed actors Jason Alexander and Mimi Rogers, sources said. Unknown commodity Some NBC exex are apparently cautious about going to affils with such an unknown commodity, fearing station exex might drop web programming in favor of syndicated fare. Letterman will host his last episode of “Late Night” June 25 and will begin his new CBS show in late August. NBC wants to have its own post-”Tonight” entry (the name “Late Night” is expected to stay) up and running by then and will likely air Letterman repeats until it launches the new show, though sources say there’s been mention of acquiring episodes of “The Tonight Show Starring Johnny Carson” to fill the breach. Both NBC and O’Brien’s representative, Gavin Polone of United Talent Agency, declined comment. Having previously announced a mid-April target date for choosing a replacement, NBC brass reportedly hopes to have a successor to Letterman set by early next week.