A bloc of San Joaquin farmers tentatively endorsed the Delta tunnels project Thursday, becoming the first significant agricultural group to support the struggling plan. But the level of support from members of the Kern County Water Agency, which serves much of the $7 billion-a-year farm economy at the southern end of the valley, was less than wholehearted. An estimated 48.5 percent of the agency’s water users said they’re interested in helping pay for the tunnels, which works out to about $1 billion in financial support.