Hey- You have great electronics going in there, so u won't have any issues there. When i was replacing the firewall on my wrecked one, i used really good 5 ply- 1/4 " aircraft plywood to cut the firewall, and then had to add another 1/4 " X 2 1/2" block behind the aluminum motor mount and that pretty well made the cowl fit perfect, and prop washer location right on location. You can go back to my thread and look at the way my firewall came out. The muffler u have might be the Slimline product- they are excellent mufflers, and i try to snatch them up whenever i see them for sale. I still have had good luck with the Bisson units, so no complaints there.
The CG should be right on according to the instruction manual, so check it with their specs, and recommendation. I don't think u will have a "tail heavy" situation with the mods ur doing with the added wood, as mine came out pretty much perfect, with a little nose drop when i picked it up on the CG. You can always move some of ur electronics, and battery more forward if u do need a little more nose weight.
Let us know how all the parts look when u unbox. Please post some pics when u start. I am lucky enough to have another brand new one in the box. and I'm already seeing "Want Ads" for them. Dean