The Falcon 56 was updated more than once. This kit is made from the Mk II plan.
Goldberg was notorious for using odd wood sizes. My best guess is that they figured you would be more likely to buy a new kit after a crash than build from the plan if the wood sizes were weird. My kit has standard wood sizes. The main spar is 1/4 x 3/4 capped with 1/8 x 1/4 spruce or bass. The rear spar is 1/4 x 3/4.
I’ve made two versions of the plywood file. I can cut it with engine beams as shown on the plan, or without beams in case you want to use a nylon mount. Let me know which one you want.