New proposals for extending the MBTA Red Line include making a pedestrian mall out of a section of street connecting Mass Ave to Brattle St. and constructing a tunnel under Mass Ave at Johnston Gate.

The Plans are tentative and will require approval of Cambridge and federal authorities.

Architects for the MBTA have submitted several versions of the proposal to the Urban Mass Transit Authority (UMTA), Mila Dixon, special assistant for communications for the Public Information Office of the MBTA, said yesterday.

The MBTA has asked UMTA for a $450 million grant to complete planning and begin construction, Dixon said.

She added that although the move would dislocate some business, the MBTA has not yet notified firms.

Final planning will start as soon as UMTA rules on the grant request, Dixon said, and actual construction would begin a year later if UMTA and the Cambridge City Council approve the plan.

Plans for the transition to the remodeled station would place a temporary station at the Eliot Street MBTA yards across Boylston St. from Eliot House, Christopher Dame, a land use planner for the Cambridge Development Department, said Wednesday.

Dame said that earlier schemes did not include the tunnel under Mass Ave, and that planners included it at the request of Harvard community affairs officials.

Donald C. Moulton, assistant vice president for community affairs, said yesterday that Harvard became involved in the planning when the MBTA asked community groups for suggestions and comments.

Moulton said the University is concerned about "ways to eliminate automobile-pedestrian conflicts in the Square."

Neither Dame nor Dixon would hazard a guess on when the UMTA would decide on funding, but several MBTA newsletters have put approval for the grant in April and completion of construction in 1982 or 1983.

Although planners have still not determined where the extension will end, Dame said it may go as far as Arlington Heights