In an odd twist on modern economics, conservationists want to use the savings and loan debacle to protect the largest privately owned old-growth redwood grove in the world. The 3,000-acre Headwaters Forest of northern California is owned by Pacific Lumber, which was a family business until it was taken over in 1985 by junk bond financier Charles Hurwitz. Hurwitz's company, the Houston-based Maxxam Corp., quickly began leveling the forest to pay off the debt incurred buying Pacific Lumber. Maxxam was also an owner of United Savings Association of Texas, which went bankrupt several years ago in one of the most expensive S & L failures in U.S. history. The Federal Deposit Insurance Corporation is now deciding whether to pursue a claim of about $500 million against United Savings. Since that's roughly what the Headwaters forest is worth, why not, conservationists ask, trade the debt for the redwoods? A bill before Congress authorizes the federal government to buy the Headwaters grove, along with 41,000 other acres of Pacific Lumber's redwoods, but it does not address the question of forgiving the debt for getting the land.