Wall Street analysts reacted favorably yesterday to the proposed merger of the food manufacturing giants Standard Brands and Nabisco. The stock market itself reacted ambiguously.

Nabisco stock fell slightly in price, while Standard Brands rose.

"This is an extraordinary favorable merger," said Alan Greditor of Drexel Burnham Lambert Inc., the large brokerage firm. "It's rare that you can find two major corporations that fit so well. It is one of the few cases where a merger will enhance the growth of both companies."

Late Wednesday, Nabisco, the nation's premier cracker maker, and Standard Brands, which numbers Fleischman's margarine and Baby Ruth candy bars among its extensive list of products, announced that their respective boards of directors had approved a tax-free merger of the two companies into a new firm to be called Nabisco Brands, Inc.

The combination, if approved by stockholders of both companies, will result in the third-largest food manufacturer in the country, with combined 1980 sales of nearly $5.5 billion. Only Beatrice Foods, with sales of $8.3 billion, and General Foods, with $6 billion, are bigger. Last year Standard Brands had sales of $3 billion, while Nabisco's revenues were $2.57 billion.

However, Nabisco's profits were $127.8 million, while Standard Brands earned $104.4 million.

Analysts said that Standard Brands will benefit from the strong national distribution network developed by Nabisco, while the solid international distribution network of Standard Brands will aid the recent efforts by Nabisco to become more important abroad.

Furthermore, an important part of Standard Brands business is the manufacture of corn syrup. Profits in the corn mill business are cyclical because price depends on the price of sugar.

"The new company will be much larger. As a result, the impact of the corn business will have a smaller impact on the overall company," according to one analyst. Experts estimate that the corn mill business accounts for about 20 percent of Standard Brands revenues and would account for less than 10 percent in the new company.

Furthermore, according to Drexel's Greditor, Nabisco is a cash-rich company, while Standard Brands is somewhat pressed for cash. The merger should permit the new company to expand into other areas.

Although American industry has been engulfed in a wave of acquisitions and mergers, the Standard Brands-Nabisco pairing seems to be qualitatively different than other recent attempts at combination. Prudential Insurance, the nation's biggest insurer, acquired the big brokerage house Bache Group Inc. outright. As brokers go, Bache is big. Compared to Prudential it is small.

Other recent merger attempts have been among financially disparate companies: the unsuccessful tries by Seagrams's to acquire St. Joe Minerals and Standard Oil of California to merge with Amax. This week's friendly merger between American Express and the brokerage house Shearson Loeb Rhoades will result in Shearson becoming a subsidiary of the $5.5 billion American Express.