DALLAS, JULY 6 -- The founding family of Southland Corp. said today it will sell almost all the company's assets except its 7-Eleven convenience store chain to finance a proposed $5.1 billion leveraged buyout.

Among the subsidiaries on the market will be Southland Dairies, one of the nation's three largest dairy processors, Chief Auto Parts and Reddy Ice, the world's largest manufacturer of crushed ice.

Southland will retain its 50 percent share in Citgo Petroleum Corp., which it co-owns with the national oil company of Venezuela. With 3,556 outlets offering gasoline, Southland is the largest independent gasoline retailer in the nation.

Proceeds from the asset sales will help finance the two-stage proposed buyout, which began today with a $77-a-share tender offer for about two-thirds of the company's common stock and all of a convertible preferred issue.

Disposal of the assets also would allow Southland to concentrate on 7-Eleven, said Jere W. Thompson, Southland's president and one of three sons of its founder, the late Joe C. Thompson Sr.

"We want very much to sell all those operations in an orderly fashion as ongoing, profitable businesses," Jere Thompson said.

Southland said it would retain its convenience operations, which include 8,222 7-Eleven stores, High's dairy stores and other units. It also will retain its Southland Distribution and Food Centers.

Going on the market will be Southland Dairies, which has 10 regional dairies and distributes products in 46 states; Chief Auto Parts, which owns 465 stores, primarily in Texas and California; Tidel Systems, which manufactures cash dispensing units, underground gasoline tank monitors and other equipment for retailers; its Snack Foods Division; Reddy Ice; Southland Food Labs and several other smaller operations.

Analysts had predicted Southland might have to sell substantial assets because of expenses involved in the stock repurchase and plans to refinance half of the company's existing debt, estimated at $1.99 billion as of March 31, 1987.

The leveraged buyout was announced late Sunday by Thompson Co., which includes John P. Thompson, Southland's chairman, Jere W. Thompson and Joe C. (Jodie) Thompson Jr., a Southland director.

The Thompson brothers own 4.9 million of Southland's 48.8 million shares -- just over 10 percent, company officials said.

Most analysts contended the $77 offer was about $7 per share or 10 percent more than what they estimated as being the value of the company.

"From an asset point {the company has vast real estate holdings as well as a large food distribution system}, it is a good offer," said one analyst. "The company's earnings are not that great because most of the stores are in difficult areas like Texas and Louisiana.

"The family is so involved in this, it just doesn't want to give it up. These guys have so much money already and don't need an additional $100 million or $200 million from someone who may take it over. There must have been a great deal of family consideration."

Another analyst, Carol Palmer of Duff & Phelps, said the family may also have been concerned the company might be put "in play" if it did not act soon. She said the $77 offer would be especially attractive to institutional holders.

"And by buying the convertible preferred, the family may want to redo the right-hand side of the balance sheet. We see it as a complete overhaul. They want to make it as attractive as possible and keep the company within the family," she said.

She said if the offer fell through, the stocks would likely plunge on the basis of the company's earnings, which are not attractive.

Southland soared on the New York Stock Exchange yesterday after trading was resumed following a 45-minute suspension on the opening. The common closed at $75.75, up $7.125; Southland convertible preferred series A gained $8.125 to close at $89.625.

Under the Thompson proposal, a tender offer was begun today for 31.5 million shares or about two-thirds of the 48.8 million shares outstanding. The company also offered to buy all the 2.5 million outstanding convertible preferred shares for $90.27 each.