The man who once headed the list of candidates for head of the Veterans Administration took himself out of consideration for the job yesterday. That may have put President Reagan back a square in filing the top spot at the $24 billion-a-year agency, but it seems likely that the administration was already having some qualms about the candidacy of James H. Webb Jr.

In a letter to Reagan, Webb, who is minority staff counsel of the House Veterans Affairs Committee and author of the critically acclaimed novel "Fields of Fire," cited personal reasons for his withdrawal. But there clearly were other reasons, foremost among them the question of who would set policy in the VA -- its administrator or budget director David A. Stockman.

Veterans' group have complained about budget cuts being proposed for the agency while it is without an administrator. Webb, who shares some of their concerns, said he had made it clear to the White House that "I wanted appeal rights beyond Stockman" -- to presidential counselor Edwin Meese III or to the president himself. The White House gave him neither yea nor nay on that.

Webb said he also was concerned that many of the Schedule C spots in the agency were being filled despite the fact the administrator hadn't been named. And he probably didn't bolster his popularity at the White House when he suggested that, as administrator, he might find it necessary to fire some of those people.

"The good news is I might have done well as VA administrator," Webb said with a chuckle. "The bad news is I probably wouldn't have lasted very long."