I know exactly what you"re talking about. It's a little nipple coming off the plenum towards the firewall. My last 95' had it capped off, and my current 95' didn't have it capped off. There's nothing that's supposed to be connected to it. My best guess is that it's for an accessory such as a vacuum gauge, which could be useful for monitoring or problem solving. If you just want to cap it off, I would recommend going to an auto parts store and looking in the "Help!" isle for vacuum caps. It's only a few dollars for a dozen or so different sizes. That's where I got mine.

As for the smoke at start up. I can almost guarantee you it's your valve seals. Both trucks I've had, have had that problem. If you don't take care of it soon, it'll end up blowing smoke even while driving down the road, which can get quite embarrassing.