I'm in the process of a similar swap with my 2000 k3500, except going 8.1.
My thought process was tackle the computer side first by swapping in the gen 3 ECU that ran 99+ LS1s, gen 3 v8's, and the 01/02 full size vans that ran the same l31 your truck has, commonly referred to as the 411 swap. I swapped one in my truck and just need one more step - getting the gas gauge working which requires a ground reference line to the sending unit, as our trucks have the reference grounded to the frame.
No need to go aftermarket for gauges, except i'm adding fuel pressure and wide band(s) at some point.
GMT400 has a number of guys that have performed this computer swap, and one member (lextech if I remember correctly) has a spreadsheet of how your wiring needs to be swapped from A to B.
My truck woke up a bit since the computer swap, but it's still not up to the duty it gets put through.
Make life easier, look for a 99-02 6.0 - it's drive by cable vs drive by wire. Otherwise you need to install the DBW pedal and associated harness and brain box.
What are you doing with the 350 once it's out?