RE: framing question?
I would assume 1/8" square but that's only from not seeing the actual plans. Is this one of Jerry Dunlap's plans from the RC Store? Look at the schematic/blueprint and it should show on that exactly what they/he mean. Eighteen inches is a little small for most nitro applications, 24" would be better. Maybe you can get it blown up. That's going to make everything a tight fit in a hull that small. I've found it tight to get a .12 and all of the gear into a 24" hull. An 18 incher would only need a small "1/2 A"/.049-.74 to move it.
I had years ago a Dumas Short Stuff that had a .12 in it but it was a shoe horn fit. Everything had to go in and come out in a certain order, no two ways about it. Somebody else built it.