The short kit comes with two firewalls. One has notches for the engine rails as shown on the plan, and the other is solid with no notches, for use with a nylon engine mount. You’ll receive both, so you can make the decision as you build.
When you order the kit, an alternate wing is available with 3/8″ balsa leading edges and 3/16″ square hardwood spars. The alternate wing is built with leading and trailing edge sheeting and cap strips. Space the ribs about 3″ apart, and there are no half-ribs (W1 on the plan).
Or you could get this kit with the standard wing as shown on the plan. Let me know which you prefer.