Thanks! (Please send me a PM with your mailing address!)
I did find a post online that says one should not use Diafine with old found film:http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00B7Hu&tag=
Donald Qualls Prolific Poster, feb 11, 2005; 06:36 p.m.
I'd recommend using HC-110 or similar low-fog developer. Plenachrome was a black and white film, ASA 50 or so before the change (it would be ISO 100 if it were still made now). As John suggested, it most likely would go 17 minutes in D-76 stock solution; a comparable time would be about 11 minutes in HC-110 Dilution B. And by today's standards, that's a LOT of development; modern Tri-X 400 takes less than seven minutes in Dilution B, even if you don't believe the huge change in Kodak's literature after the emulsion coating change. However, that's a time that has been reported to work well for old film here previously.
Whatever you do, *don't* develop old found film in Diafine -- Diafine will increase the fog, rather than restrain it, and leave you with film on which you can barely make out an image, if at all. Been there, done that...