Tach Dash conversion

[Silverado6.6]

Hey guys, I'm wanting to know how difficult it would be to install a tach instrument cluster w/ the 100 mph speedo, in place of my 85 mph speedo, non tach cluster. I know that the tach won't be too hard to install, should be just like installing an aftermarket tach, but I want the 100 mph speedo, and I figured that would be harder. I've got actual guages vs. idiot lights w/ both clusters, so everything else should hook up okay, also, originally the truck had the stock 9.5x16.5 tires and I went to a 235/85/16, I believe the stock ones are aprox. 29" tall and mine are aprox. 32" tall, and I've got a 3 73 gears now, and am going to 4 10....so I'm wondering what all I'm gonna have to do in order to have an acurate speedometer. Thanks

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'81 3/4 ton Silverado Warmed up 355
TH400 w/shift kit
Corporate 14 bolt w/ 3.73s