woah........
i know your excited bout gettin it for free but you gotta just breathe.......
You got so many questions all at once i dunno where to start!
First of all, if you are gonna run an LS2 in a 4x4 tahoe you are definitely gonna want a 4l80e cause it will roast a 4l60e in no time. Im not sure but if you wanna keep the 4x4 you can probably get the 4x4 4l80e and transfer case and it will atleast get close to bolting up.

As far as the dash goes, i have never done it nor heard of anyone doing it but im sure you could if you REALLY wanted to. Im sure it would be a major PITA and not worth it.

And "wanting to lift it like 6'' but i do want to lower it a bit"....good luck with that.....