The vasty territory of the Jari Forestry and Ranching Co. would hardly seem to be the ideal place to install an industrial plant. Situated along a remote tributary of the Amazon River, some 250 miles from the nearest city, Jari's 3.7 million acres can be reached only by boat or plane.

But billionaire Daniel Ludwig, owner of this Massachuetts-sized Agricultural complex in Brazil's undeveloped north, has reached deep into his pockets in a unique effort to overcome the inaccessibilty. In a project whose total value has been estimated at $450 million, Ludwig has had a cellulose factory built on floating platforms in Japan and towed across the sea to Brazil.

The first of the two units that will make up this pioneer plant left the Hiroshima shipyards of Ishikawajima-Harima Herry Industries (IHI) Feb. 1 on the intial leg of a 15,000-mile voyage around the Cape of Good Hope. The second floating platform left a week later. Both units are to arrive at Jari's Amazon holdings by early May.

There, the cellulose processing mill and its sister generating plant will be maneuvered into a specially constructed canal along the banks of the Jari River. The canal will then be drained, leaving the platforms permanently mounted on a system of wood pillars.

For the 801 year-old Ludwig, reputed to be the richest man in the world, the cellulose project represents one more way to reap profit from what is already a remarkably successful enterprise. A major cattleraiser and grower of rice and soybeans. Jari first ventured into timber farming in the late 1960's, replacing trees native to the Amazon with two faster-growing imported varieties.

The lack of infrastructure in the Amazon region, however, hampered efforts to industrialize Jari's vast timber stands. The problem remained unresolved until Ludwig, who is generally credited with inventing the supertanking, turned to IHI president Hisashi Shinto - who in the early 1950's had been chief engineer at Ludwig's Universe Tank Ships Inc.

From a series of consulations between the two men emerged the decision to build a fully equiped industrial plant halfway around the world from its final destination. IHI began construction of the platforms in March 1976 at the same shipyards where, more than two decades earlier, Ludwig had built his first supertankers.

Completed ahead of scheduled, the two floating platforms are being sent via a roundabout Indian Ocean route because they are too big to fit through the Panama Canal. Each measures 250 yards by 5C yards and weighs about 30 tons.

Ludwig's cellulose venture has been supported, at least indirectly, by high officials of Brazil's military government. As early as 1974, a policy statement issued by the state Superintendency for the Development of the Amazon (SUDAM) called for large-scale exploitation of Amazon timber resources and citied Jari as a model for other potential "integranted industrial complexes." Jari's floating platforms also have been exempted from restrictions on capital-goods imports.

Some Brazillian businessman affected by the limitations are protesting the exemption granted Ludwig Minister of Industry and Commerce Angelo Calmon de Sa said last month, however, that the Jari case is "closed."

Opoosition notwithstanding, the complex is expected to begin operation late this years, producing 750 tons of cellulose daily for customers in Finland and Brazil. Intended to be self-sufficient, it will powered by 55,000 kilowatts of electricity from a generator that will run on abundant local supplies of tree branches, bark and grass.

Despite its high price and risks involved in its transport, the floating cellulose factory is being looked upon as something of a bargin. According to press reports here, Jari officials estimate that they saved two years in time and 20 percent in costs by not having to go into the jungle and build the plant from scratch.

For that reason, some observers see the Jari complex as the forerunner of a new approach to economic development projects in the Third World.