I don't get 23mpg, i get 24mpg, hehehehe,
I have this:

ls6 block, ls1 internals, 6.0L truck intake, all truck front accessories, vette maf, built 4L60E. I really like how the truck intake looks in a truck, i'm sure the ls6 gives a little more power, but my preference is the truck intake..
I'm guessing i have about 380hp at the crank, i'll find out now that i have new tires, i'll take it to the dyno pretty soon.
To build power for the ls1 during the buildup, slap a cam in the there, maybe the gm hotcam or similar. They also respond well to a good exhaust system.
You can basically follow all old school rules for building your ls1.
It may be a little more difficult to piece your engine together like me, but i really like the way it came out.

Just from my experience, the 4L60E trannies aren't the best , but they can be, just make sure it's built to push those big tires and heavy truck. a shift kit and some clutch packs, along with a hd 2/4 band should be ok

keep the questions coming, i don't mind.