DALLAS (AP) ¿ Builders FirstSource Inc., which supplies building products, said Tuesday its two largest stockholders have proposed a restructuring of the company's debt. Shares fell more than one-third in afternoon trading. JLL Partners Fund V and Warburg Pincus Private Equity IX would restructure Builders FirstSource's outstanding $275 million principal amount of second priority senior secured floating rate notes and a stock rights offering to share owners. Combined, JLL and Warburg own about 50 percent of shares of the Dallas-based Builders FirstSource and approximately $98 million of the notes. The board of directors said it has established a committee to evaluate the proposal. Shares fell $2.72, or about 35 percent, to $4.97.