The chairman of the Senate Finance Committee said yesterday that Congress would be "foolhardy" to approve President Carter's proposed welfare revisions on anything more than a trial basis.

"The same types of problems that exist in the present welfare program will exist in the new one for the simple reason that we are dealing with human beings," said Sen. Russell B. Long (D-La.), whose committee will handle the welfare legislation.

"Congress would be follhardy to jump into such a vast undertaking without exploring some fo the problems that will develop by field-testing the system long enough to know what we can expect," he said.

Carter proposed Aug. 6 to scrap the basic components of the nation's welfare system and replace them with a $30.7 billion program of income supplements work incentives and public-service jobs.

Long's strong reservations about the plan, coupled with a barrage of criticism from conservative Republicans, ensure rough going for the Carter proposals on Capitol Hill.

The senator said Carter's proposals are based on "the best of intentions," and he praised the President for trying to improve the welfare system. But he said the program proposed by the administration "makes certain assumptions about the way people behave."

"The best of intentions can lead to unintended results if those assumptions prove wrong," Long said, adding that similar assumptions have proved wrong in the past.

For example, he said the Social Security disability insurance program was put into effect with the belief that only totally disabled persons would use its benefits, and that no one would misrepresent himself or herself as being totally disabled.

Instead, Long said the program has provided "an incentive for people who are not totally disabled to convince themselves that they are totally disabled in order to receive benefits." As a result, the program has cost far more than intended he said.