The Census Bureau director said yesterday the fast-growing Sun Belt will probably pick up two more congressional seats than had been previously estimated, giving that area a total of 16 new members in the House of Representatives.

Vincent Barabba, the census director, refused to identify the two states to which the new representative seats will go, but said they will be taken from northern states where population has declined over the past decade.

He said the new reapportionment estimate was tentatively made after census officials discovered they had counted four million more persons than they had expected and that the great majority of them were in southern and western states.

Barabba said the bureau expects to report later this month an official U.S. population of roughly 226 million, a growth of about 23 million since 1970.

Calling the 1980 count the most successful of all time, Barabba said the bureau has no intention of adjusting the figures to compensate for any persons missed because it believes the numbers are extremely small. A Detroit federal judge, ruling in a suit brought by Detroit and other big cities, ordered adjustments in late September, but the bureau is appealing that decision.

Barabba said the burea knows it has included some illegal aliens in its 1980 count but has no way of determining how many.

Earlier estimates of changes in congressional representation indicated that Florida would pick up three seats while California and Texas would gain two. States getting one new House member would be Arizona, Colorado, New Mexico, Oregon, Tennessee, Utah and Washington.

Under those estimates, New York would lose four seats and Illinois, Ohio and Pennsylvania would each lose two. Massachusetts, Michigan, New Jersey and South Dakota would each lose one seat.