You might want to fix one thing though, if you've got a 99-02 you can't use the '03+ harness. You'll have to switch to a cable throttle, or find an DBW 00-02 harness, which is not an easy find.
And the V6 mounts can be made to work, but you'll need a 5/8" spacer for each side, and longer bolts. However, it does move the motor/tranny back a bit. I had to slot my rear tranny/crossmember holes a little. It wasn't back enough that I've had any driveshaft issues though.
I ran into this problem when I did my swap. Contrary to what I was told, it won't work, so I'm attempting to recover some of my money by selling the leftover parts.
Out of curiousity, what did you do about the AC lines/compressor? Did you re-use the V6 one?
'99 GMC RCSB
3" Body lift, 4" Rancho lift, Fox shocks
36" BFG All-terrains
LQ9 6.0L swap, gt2-3 cam, longtubes, LS1 fans, Nelson Performance tune
4.8 gears/Eaton posi
Built 4l65e
Custom paint, roll pan, filled tail lights, relocated tail gate handle
Future mods:
Boost!!