In general the heir(s) of the writer will own the copyright unless the transfer of title documents you have for the diary specifically state that copyright was also transferred.  I can’t remember what the US situation is at the moment but in Europe this would last for 70 years after the writer’s death.   In Britain if copyright is not specifically mentioned in a will it will pass to the person who inherits the residue of the estate.  If you can track down the heir(s) you can offer to buy the copyright or ask them to donate it (they could do this outright or they could sell/give you a licence just for this publication). 

 

The museum would however have copyright in the particular transcription (as this requires interpretation of the handwriting it has original intellectual input separate from the writing of the diary in the first place).   (Assuming it was done by paid staff in work time, if it was done by volunteers or staff in their own time you would need to get them to assign their copyright in the transcription to the museum). 

 

So the publication would have two copyrights – the writers, and the transcribers (which would expire 70 years after the death of the last transcriber)

 

I don’t believe you need to “register” your copyright in the USA any more.

 

If you are in Canada there is a procedure for obtaining copyright permission if you can’t find the copyright holder.