Many solutions here. Having used ZDZ for years, just not on that big of an airplane, the challenge is easy.

First, if the existing firewall to spinner backplate dimension is LESS than the overall length of the motor (including the carb), then, YES, you must rout a hole in the firewall.

Second, many designs that I'm aware of have a firewall far enough back that you'll actually end up mounting the motor to standoffs. On the past three airplanes I've built, the standoff distance was enough to get the carb in front of the firewall.

Get the dimensions from Rocco for the firewall to spinner, and from Mike at RC Showcase for the ZDZ hub to rear of carb dimensions.

I'd encourage you to get the facts from the measurements.