I used to use this one frequently for my 88 GMC, but seeing as I never drive it and I'm probably going to soon sell it off I haven't been on there much recently. It was pretty informative and a lot of those guys have parts and good knowledge.
Only caveat is there seems to be a much higher proportion of redneck "pseudo-racer" Nascar-hat wearing types on there than the Porsche sites though (or at least there used to be) - that's the only turn-off I ever found on there - overall it's a good board.
http://67-72chevytrucks.com/vboard/