Salomon Bros. economist Henry Kaufman yesterday predicted a further decline in interest rates in 1983, assuming that "we can get through the year with just modest economic expansion, and a continued slowing of the rate of inflation."

Kaufman, whose forecasts of interest rates are closely watched by the investment community, added that real economic growth could expand by as much as 4 percent--slightly more than the administration's current official estimate, but just under revised unofficial guesses--without regenerating inflation.

Speaking at the National Press Club, Kaufman predicted that home mortgage rates would come down further, aided by the flow of money into bank money market funds, which are seeking longer-term investments, including housing. Over the next several months, he said rates on fixed-mortgages might get down to 12 percent, or a bit lower.

The major theme of Kaufman's speech was that further declines in interest rates are crucial "if business is to make a significant contribution to economic expansion in 1984 and beyond." And with Fed Chairman Paul A. Volcker sitting at the head table, he praised what he described as a recent switch by the Fed from preoccupation with money targets "to a more judgmental approach."

"Inflation should no longer be the overriding concern," Kaufman said. He cited as reasons the decline in inflation that has already taken place, and which should be extended by lower oil prices, moderation in wage settlements and the weakness of commercial borrowing caused by large existing debts.

He credited the Fed's "new approach," which relies on judgment rather than strict adherence to monetary rules, with checking "waning confidence" in the banking system and calming down the credit markets.

Kaufman said President Reagan "should" reappoint Volcker when his term as chairman expires in August "for a variety of reasons." He said Volcker had played a critical role "in trying to arrest a very serious malaise in the United States and globally." But beyond that, Kaufman said, "we continue to be in an exceedingly volatile period of transition requiring good guidance and expert knowledge, both domestically and internationally."

Now that the Fed has moved away from an earlier, rigid approach, Volcker's knowledge of the "nuances and the techniques of financial markets and monetary policy is even more essential than in the past. Therefore, I would hope that the president will reappoint the chairman of the Federal Reserve Board," the New York economist said.

But the precise nature of the Fed's policy approach remained, as usual, a bit fuzzy. In testimony Tuesday, Volcker had said that M1, the basic money supply measure, had been rising "somewhat more rapidly than I would have expected."

This was given a bearish interpretation, briefly, on Wall Street, which read into that comment a hint that the Fed would act to slow the rise in the money supply. At a meeting yesterday of the National Association of Manufacturers, Volcker reiterated that "we have to return to a more ordered money supply growth pattern."

But he carefully avoided saying that the Fed would take steps to slow the growth in the money measures, observing instead that during the present period of "institutional change"--creation of new money market accounts in the banks, for instance--the Fed must display "a certain amount of assessment and judgment as we move along."

Another NAM speaker, Council of Economic Advisers Chairman Martin Feldstein, predicted that money supply growth will taper off after the change-over by some depositors to the banks' money market accounts is completed. Feldstein said monetary growth could not continue at recent rates, and that Fed policy should aim at a nominal growth of 10 percent or less in the gross national product.

Feldstein went out of his way to prepare the public for some February economic statistics that will be less encouraging than January's, which in some cases "seemed to good to be true." Nonetheless, he said a recovery was under way, and will be stronger than the 3 percent gain the administration had suggested in January.

In his sweeping analysis of the domestic and international economic scene, Kaufman said he hope that "fiscal policy will take the same turn" toward "the new judgmental approach" that he said had been adopted at the Fed.

Because of what he said were the grave implications of the huge prospective budget deficit, he called for abandonment not only of the tax indexation scheduled for fiscal 1985 but also of "all government and private indexations against inflation." For the same reason, he said he would approve of a $5 to $10 per barrel oil import tax, but not until 1984.