It sounds to me like incomplete combustion as a result of a cold engine. Have you done a compression test? Do you have a video of the smoke you can post on YouTube. Almost all old diesels will smoke some when cold. My 3208 will lay down a smokescreen large enough for a battalion when cold but is clean once it warms up.
I certainly wouldn't go to the work of replacing valve stem seals without further confirmation they are bad if you are getting gray/white smoke when cold.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13