CG Ultimate
The previous post pretty much covered the pros and and cons of moving the firewall. There is another mod you really should consider if you haven't built the wings. Get you some 5-ply 1/8 plywood and duplicate the four ribs which are the attachment points for the outer cabanes. The light ply ones in the kit will not take very much abuse. If you put a couple of lightening holes in the new ones you really want pick up that much weight. Then after you build the cabane add a short piece of wood dowel to the location where the 4-40 bolt goes thru the cabane. This will keep the cabane from being crushed.
You might even want to consider replacing the light ply on the firewall with the 5-ply, just a thought.
Also just so you will be thinking about it. If you want the muffler to be inside the cowl, you will have to do one of two things. Cut a half moon shaped tunnel in the bottom of the firewall and chinblock. Then face it off with some 1/64 ply that you can easily bend to the curve of this area. Your second choice is to cut a small v in the exhaust header, bend it to a greater angle then mig it (weld) it back together.
I've had 120s and 150s in mine and I have shortened the firewalls, added two degrees right thrust and put small servos under the horizontal.
Good Luck
Gary