[Your41Plague12]

The plugs should make some difference. I'd also check the vacuum lines; it seems that when you shift the transmission into gear and the difference in vacuum is created, something gets funky there. If you have a TH400, check the vacuum modulator. Mine needs replaced too. For more power, definitely hook up the secondaries. Judging by what this website says, it looks like you have what they cal "Ash (Lead)" deposits building up. This website describes the symptoms and what can be done to fix it.

http://www.classictruckshop.com/club...park/plugs.htm

It claims it can cause minor misfiring, which would explain it running rough. It seems to me like replacing the plugs would be a great starting point. Get back to us once it's done and maybe we can come up with a different conclusion if that doesn't fix it.