What you need to do is measure the the length from the firewall to the prop hub. That will determine your spacers for the motor so that you have the proper distance between prop and cowl.

Next if the carb is hitting the firewall you neeed to drill a hole in the firewall to allow the carb to stick through. Make it large enough that you don't have to remove the carb.

I've had to do this with all my 3W55's because the carb extends back much farther than most motors.

Just spend the time to fuel proof the inside of the firewall as well as the carb will spit fuel (even if its a small amount) and over time it can weaken the wood.