Welcome, Scott, to the world of the mentally unbalanced......I'm working on a Zenith 601, but the voices in my head are shouting "P-40" in my brain, too. Call it monomania, but when I get the 601 done,..................... What I have in mind, though, is a Light Sport, of sufficient performance to have fun, and maybe do a little cross-country. My particular madness went something like this:
Guillow makes a 28-inch wingspan balsa model of the bird, with wonderfully detailed plans. What I'm thinking of is to enlarge the plans via overhead projector, 12:1, and thereby have plans for a 28-foot wingspan model. That would make it roughly 70 to 75% scale. Using the plans for their balsa bulkheads, I'm looking at making forming blocks for ONE SIDE of the bulkheads, with the addition of a 2 or 3 inch tab extending from each end of the halves. Tap a joggle on one side, then rivet them together, and, voila, a bulkhead. String them on two tight cables, top and bottom. (the holes would have to be very carefully aligned) and stretched on about a 15 foot 2x4 frame, and they'd then be supported well enough to hold until placement of four longerons, tail to cockpit, and cockpit to firewall. I'm thinking of using one by one angles, eighth inch thick extrusions. From the cockpit back, four skin sheets: bottom, sides, and top, with J-bends at the upper edges for strength, all done with Avex rivets. I doubt if I can English-Wheel accurate curves for the fuselage, so maybe multiple pieces of skin will have to do.
The Guillow plans have very good drawings of the ribs, too, although I'm not sure if they're really the right airfoil. Probably wouldn't be too hard to find out. Then, using somewhat the same construction system as Zenith's wings, an 8-foot center section, and two 10-foot outer panels in the P-40 planform ought to make it "P-40-ish" Power? Whatever will give around 100 horses ought to get it up to 128 MPH max cruise, LSA parameters. An old aluminum V-6, inverted, would give direct drive, and authentic looking exhausts. Since LSA allows 1320 pounds, a one-seater could still make the weight. If LSA isn't a consideration, you could cram a lot more horsepower into it.
I think the horizontal and vertical stabilizers would need two spars for adequate strength. The tips, as with the wingtips, would be a challenge, Maybe fiberglass over hand-sculpted molds.
I have no clue what to do for landing gear. I looked at a P-40 at Oshkosh in July, and understand the gearing that makes them pivot, but don't have the faintest idea how to work them around the main spar. May have to settle for stiff legs.
I haven't checked with Loehle, but if they would part with a cowl, it should be an almost perfect scale fit. Anyway, that's what's been rattling around in my brain, My EAA chapter has enough guys with real talent to help get it done when (if) I finish the 601.
Re your questions, the "lowest bidder" system probably didn't set in until later. I'd guess they meant it to take battle damage, and 12 or 13 ribs per wing would be just fine to me. Good luck in your planning/building. I'd be interested to know how you come along with the project. If you're not already a member, I'd encourage you to join the nearest EAA chapter. They have tons of experience, and are always ready to help.