General Motors Corp. now will eliminate 4,000 salaried positions this year by Oct. 1, or 600 more than the bankrupt auto maker previously planned, as it works quickly to restructure its workforce to match its slimmed-down business.

Salaried employees received letters Tuesday detailing the cuts, which include the option to quit and receive six months of pay with benefits or retire early with full benefits.

“We’ll first be asking for people to raise their hands by Aug. 3, and then we’ll take those people into consideration as we finalize the plan,” says GM spokesman Tom Wilkinson.

GM wants to trim its U.S. salaried workforce by a total of 6,150 people before year’s end. The cuts include a reduction in its executive ranks of 34%.

“There will be significant change,” GM President and CEO Fritz Henderson said of the executive turnover in a recent web chat with journalists.

A caveat of the auto maker’s government-funded bankruptcy calls for GM to sharply reduce its number of executives.

GM wants to reduce its total U.S. employment rolls from an estimated 92,000 people in 2008 to 64,000 by the end of the year, which would reflect the auto maker’s elimination of four brands, the closure of several manufacturing plants, hourly attrition programs and a reduction in its number of managers. The cuts would leave GM with 23,500 salaried workers and 40,500 hourly jobs at the start of 2010.

Earlier this year, GM said it wanted to reduce its entire global employment this year by 47,000 to roughly 188,000, but Wilkinson says those projections would likely change given its attempt to sell off a majority stake in its Adam Opel GmbH unit in Germany.

GM began the decade with 390,790 employees in the U.S. and 595,000 worldwide, according to Ward’s.