I would like to change my Tahoe from automatic.

It will be more of a hassle than you can imagine. Not only will you need a diffent computer, weld in the proper floor board around shifter and tunnel, modify wiring harness for different sensors, etc.
You may still have speedomteter problems, you may neeed a new tranmission crossmember.
It is such a custom change that the best place to get information is from someone who has done it. Your best chance to find someone like this is at an enthusiast forum for your truck. There will be a few out there so you may join more than one.