You want to be careful when changing too many things at once, you might end up with a truck that falls on its face........but looks good on paper.
here is a link to some information that we published on this forum, you will need the transmission, rear tire diameter, and then know what rear axle ratio you currently have.
With these spreadsheets you will be able to see how much torque you currently have to the ground with your current set up, then compare the speed lines and available torque before you change the rear axle ratios or transmissions.......post me here if you have any questions.....Kieth
here is the link
http://6066gmcclub.com/forum/showthread.php?t=48090