Lawmakers are promising to iron out House and Senate differences over a $6 billion drought-relief package in a quick flurry of action next week and speed the measure on to President Reagan's desk.

"My opinion is that the president would accept this bill as it is passed today, although there have been some things he objects to," Sen. Richard G. Lugar, R-Ind., said Thursday after 94-0 Senate passage of the bill.The House had approved its version 368-29 a few hours earlier after scaling back a plan to increase milk price supports, a proposal widely criticized as excessively favoring the dairy industry.

The revised version would impose a temporary increase for three months next year. Milk price supports previously had been scheduled to decrease on Jan. 1.

Senators tightened up a similarly controversial benefit in their bill that would allow companies that make ethanol for fuel to buy corn from government surplus bins at what could shape up as a substantial discount.

However, there were still warnings that Congress was flirting with budget trouble by inserting special benefits for constituents back home into the measure.

The House, urged by Agriculture Committee Chairman E. "Kika" de la Garza, D-Texas, acted to cap the price tag of its bill at $5.8 billion.

The Senate had no comparably firm figure. Sen. Phil Gramm, R-Texas, said latest estimates put the cost of the Senate measure at between $5.85 billion and $6.55 billion.

Gramm warned that running over a $5.8 billion ceiling calculated under the Gramm-Rudman budget law would set off nasty fiscal and political repercussions.

At the heart of the bill are provisions that would provide disaster payments to farmers equal to 65 percent of losses beyond 35 percent of expected harvest. Many dairy and livestock producers would become eligible for benefits under an expanded government feed aid program.

Farmers especially hard hit by the drought would get supplemental payments under both House and Senate bills.

A number of other points remain to be resolved when lawmakers meet in a House-Senate conference committee next week. The House version requires farmers to buy federal crop insurance for each of the next two years if they are to be eligible for disaster payments this year.

The Senate version does not contain such a provision, which is opposed by both the Reagan administration and a wide array of major crop producer groups.

Both bills would cancel a 50-cent cut in the milk price-support level scheduled for Jan. 1, but the House version would go beyond that. The House provision would raise the price-support level from April through June 1989 by 50 cents, to $11.10 cents for each 100 pounds of milk.

Dairy-state lawmakers wanted even more, pushing for a permanent increase.

"This is a most dangerous and wrongheaded precedent," Rep. Pat Roberts, R-Kan., said in opposing the measure. He said it was unfair to provide such aid to dairymen and not also to cattle, sheep and poultry producers, who must pay the same increased feed costs.

The House, however, voted 217-181 for the temporary increase, a compromise sponsored by Reps. Steve Gunderson, R-Wis., and Tim Penny, D-Minn.

House conferees are going to be looking hard at a Senate provision that would allow the nation's ethanol makers to purchase surplus grain from government bins at 110 percent of the Agriculture Department's acquisition cost. Much of the government corn was acquired long before the drought pushed up prices.