John,
I have replaced all of the wood work on my 41, but I did it when the cab was off of the frame. You could remove/replace that piece with the cab on, but it would be difficult as the wood is sandwiched between the cab and the 2 cab channel iron pieces which bolt to the frame. You may try unbolting the front body mount, removing the ~6 bolts attaching the cab to the channel iron (all the way to the back of the cab), and then carefully prying to separate the cab and channel apart. Don't pry too hard or too high though as there is not much structural integrity between the cab and the firewall when you have it this far apart. Crazy prying/lifting could distort your windshield A-pillar which would affect your windshield alignment.
Make sure to use a decay resistant wood for a replacement as you will not want to do this twice.
Paul
