Since the pilot had filed a VFR flight plan he was not reporting his progress or being tracked. That was a bad idea BTW since weather reports called for fog and icing conditions.

In all these years the plane has not been found, and since they were flying a coastal route in mid-October they should have been found. The snowpack was not that thick yet. Since they were not found immediately then they should have been found after the spring thaw or the after next spring thaw.

Since they haven't been found after 50 spring thaws during global warming, we can conclude they crashed in the water, likely through thin ice, and sank.

Thin ice would have kept buoyant wreckage hidden and unspotted until it was too late to associate it with the crash.

Thin ice usually develops in coastal Alaska in lakes in mid-October and a few weeks later in sea water. So I'd bet on a lake and probably a deep lake in the route.

If I were to be making a sonar search I'd start with Malaspina Lake. It's 145' deep and usually closed to netting. Too deep for sport divers to find the plane, too heavily regulated for fisherman to bother with, and iced over thinly just when the crash would have occurred. It's heavily stratified making a bow mounted sonar ineffective and side scans in 1972 were very primitive.

Today's tech could work but nobody would take on the job without a lot of funding and its likely no possible sponsor thinks the risk reward is sufficient to try.