* Ackman, of Pershing Square, seeking 5 board seats

* Says keeping personal stake at least 5 yrs if wins seat

* Target shares unchanged after hours (Updates stock, adds Pershing call for voting protocols)

By Nicole Maestri

SAN FRANCISCO, May 26 (Reuters) - William Ackman of Pershing Square Capital Management, fighting for seats on the board of Target Corp (TGT.N: Quote, Profile, Research), asked the retailer to allow its 250 largest shareholders to split their proxy votes between his slate of dissident directors and its incumbent nominees.

Pershing Square, in a filing with the U.S. Securities and Exchange Commission on Tuesday, suggested the two sides post a letter online that the top 250 shareholders -- representing more than 75 percent of Target's outstanding shares -- could use to split their votes.

"That letter would confirm precisely which nominees each of these shareholders seeks to support," Pershing Square stated.

Ackman runs Pershing Square Capital Management, which has amassed a 7.8 percent stake in Target. He is seeking to win five seats on the retailer's board at its annual meeting on May 28, while Target is running a slate of four incumbent directors.

The board has 12 members. Ackman says it should have 13, an issue that will be voted on at the meeting.