Silverado6.6
To correct your speedo gears go to this website. You can type in your tire size and other information to find the gears you need.
http://www.tciauto.com/tech_info/speedo_gears.htm
As for installing the 100 MPH and tach the swap should be very simple. Your best option is to abtain the green ciruit board that the 100 MHP/tach were original removed. The green ciruit board will have the 2.5 inch gauge clusters wired correctly. In particular the fuel gauge since I'm assuming your truck came with the large fuel gauge on the righthand side. I've never done the swap but I have torn my truck apart enough to notice a ton of interchangeble parts from other years. Like digital guages? Check out my web page. I replaced the entire four small gauges with digital displays including a tach.
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Gerardo
1983 Custom Truck
TX Plates: "BAD BOW"
http://www.geocities.com/abetterchemist/