[nxtruck]

Guys,
I'm in the process of doing just this to my truck. I'm using a pump from a '00 Camaro SS. I mounted and wired the pump this past weekend and am waiting on the breather tank and oil cap. From what I've heard, you can expect anywhere from around 2" all the way to 8" of vacuum from one of these pumps. I think the 8" quote might be a little optimistic, but we'll see. I've got a vacuum gauge that will be plumbed inline, so I should be able to monitor how much vacuum it's pulling.
It's been my experience that the header-mounted evac tubes do little, if any, on a vehicle with any sort of exhaust system. On an open-headered race car, they can be made to pull around 4 or 5"s of vacuum, but only if the evac tubes are positioned perfectly in the collectors.
My main concern is pump life. I street-drive my truck quite a bit, and don't want any reliability problems from this setup. Time will tell, I guess.
One more thing to consider: even if these pumps will only pull 3-4"s of vacuum, that's 3-4"s more than you had using just v/c breathers. As far as power is concerned, it's all dependant on how efficient your pump is and how well your engine is sealed up; internally, that is. An efficient vacuum pump will help with any oil leaks you may have, and will also help to control windage in the lower end.
I'll let you guys know how this works once I get some miles on it.