No way can you build a mount out of balsa, will be way too weak. You will need the mount beams made out of hard wood (see attached photo), and the ply plate that screws onto the bearings (recommended) will need to be 1/8" thick or a bit thicker. Reinforce behind the firewall with plenty of tri-stock and it may be strong enough to handle the vibrations of that engine. My first choice would be to sand the engine bay to make it wider for the engine if you can without making it weak.