I finally got this bugger taken care of. Thankfully we got some unseasonably warm weather here and I could get outside a few times to deal with it, plus the fact that when I first tried the install, my pellet chute snapped right off as it turned out to be rotted as well and I had to order/wait for that to get here first...
Anyway, I've included a picture of the completed project.
Here's a few tips if you have to do it yourself:
1-VERY SHARP HIGH QUALITY drill bit(s) are REQUIRED!! Drilling thru that stuff is HARD and I broke 2 bits before completing with the 3rd dull when done.
2-I needed to get a 90 degree angled chuck attachment for my drill as I could not fit drill bit + drill gun into the small area at proper angles to drill holes and secure the screws.
3-Make sure you install the chute side wall halves FIRST. If you do the back and front walls first like I did, you wont be able to insert the chute wall parts as the flame bar mounts will get in the way and you will need to subject them to some bending:-)
4-Make sure when installing the back wall that it is high enough to clear the ash pan underneath. Mine is a little tight. The same for the side walls, but its less of an issue as the width of the box is wider than the tray itself.
5-the pellet chute can be removed w/o taking the entire hopper apart. Just remove the two screws in the hopper holding the chute, and while lifting/pulling on the chute turn on the grill so the auger spins. This helps get it unhooked from the auger (do the same in reverse to install the new part.)