The Reagan administration's controversial choice to oversee national forest management, blocked for more than a month in the Senate, is in more trouble on Capitol Hill.

Rep. Jim Weaver (D-Ore.), citing new concerns about conflicts of interest, says he finds it "inconceivable" that John B. Crowell Jr. has not withdrawn his name from consideration as assistant secretary of agriculture for natural resources and environment.

In a House speech and later interview, Weaver, who is chairman of the House forestry subcommittee, said, "It is simply incredible that someone who comes from a company that has carried out an almost classic cut-and-run timber operation can seriously be considered for this sensitive post."

Crowell, former general counsel for the Louiasiana-Pacific Corp., a major timber cutter in federal forests, responded that he sees no conflict in his nomination.

The Senate Agriculture Committee approved the nomination March 31, but it was blocked from floor action by Sen. Edward M. Kennedy (D-Mass.), who had questions about Crowell's qualifications to oversee the U.S. Forest Service. Another "hold," blocking a floor vote, was invoked last week by Sen. Patrick Leahy (D-Vt.), who said new and more troubling questions about Crowell's past must be answered.

Crowell's latest problem involves his role with a Louisiana-Pacific subsidiary found guilty by a federal judge of conspiring to fix prices and control the timber market in Alaska's Tongass National Forest, the largest in the country.

After Judge Barbara J. Rothstein ruled orally in March that Ketchikan Pulp Co. was guilty, the Forest Service clapped a three-month freeze or new timber sales there while it investigates the company's conduct. Existing contracts were not affected.

Rothstein has not yet issued a written order, but the firm -- now known as Louisiana-Pacific, Ketchikan -- has announced that it plans an appeal once the ruling comes down.

Crowell was general counsel of the parent company during the 1973-1975 period when the alleged conspiracy took place and participated in some of the defense preparations before the case went to trial.

Assistant secretary of agriculture, he would be in a position to influence Forest Service handling of the investigation and would have the final say if the department decided to sue Louisina-Pacific, Ketchikan or ban it from future sales.

Crowell said in an interview Friday that he has avoided the matter as assistant secretary-designate and would not become involved with the government's handling of the case.

"The Forest Serice and I have agreed that anything they do with the review is something that they and I don't talk about. I couldn't get involved. It would be a conflict for me," he said.

During his nomination hearings, where he was bitterly opposed by conservation groups, Crowell gave similar assurances to Sen. Walter (Dee) Hudleston (D-Ky.), who queried him at length about potential conflicts and past activities.

Crowell promised that he would remove himself from any specific appeals brought by his old company. And, he said, he would stay out of any federal pesticide regulatory actions involving American Home Products Corp., in which his wife holds stock.

Until Weaver and Leahy raised questions about the Alaska conspiracy case, most of the objections to the Crowell nomination centered on his role as a strong advocate of more cutting of public forests by private companies. Louisiana-Pacific is the major cutter of public timber.

Weaver said his earlier reservations about Crowell were reinforced by the Ketchikan case. "I would find it hard to believe that Mr. Crowell as general counsel of Louisiana-Pacific was not involved in those operations -- the legal operations -- of its wholly owned subsidiary."

"If he didn't know what was going on, then he ran a very loose operation and he let a subsidiary run wild. The most damning thing is that he's going to be the political boss of the Forest Service, which is going to have to clean up this mess. I find it inconceivable that he doesn't withdraw himself from the nomination," Weaver said.

Crowell responded, "I wasn't in any way involved in the alleged wrong behavior of the company. If I had been, I would have been called as a witness in the case, which I wasn't. My sole role was that of a lawyer. I did nothing more than choose the defense counsel and interpret what the counsel was telling me about the case and involve myself in the tactics and strategy."

Crowell acknowledged that, as general counsel for the parent firm, it would have been this duty to stop the conspiracy if he knew about it. But, he added, "the general counsel is usually the last to know about this sort of thing."

In her ruling from the bench, Rothstein used strong language to condemn Ketchikan's actions. She said the trial "overwhelmingly" established that executives of Ketchikan and a codefendant, Alaska Lumber and Pulp Co., conspired to restrain trade and monopolize the industry in southeast Alaska.

Rothstein said the two firms began with a major advantage -- 50-year leases with the Forest Service that assured them of a vast timber supply. But, she said, they had conspired to rig bids, lowering prices paid to the government, and worked to keep smaller loggers from sharing the market.

The suit was brought by Reid Brothers Logging Co., a small independent firm in Alaska. The Reids sought $2.1 million in damages.