Greg,

yup, thin ribs indeed!

Yes, the F2 design is my least favorite part of this kit. The doublers undoubtedly made the fuse sides hard to flex. Given the added strength, I would consider two things. First, I would either build up the frame of F2 with hardwood rather than balsa. Alternatively, if it's still an option, make the entire former of ply. You can cutout a rectangluar ring for the top half and use the available bottom half for the wing dowels. Don't forget to drill out the holes in the bottom for the dowels as shown on the plans. If you haven't done so yet and your FW is not in place, it should still be fairly easy by drilling from the front.

Secondly, I would spray a good dose of windex (or anything with ammonia) on the outside of the fuse sides before gluing in the FW. Use 30 min epoxy or slower, bring the fuse sides in, tape in place, align on plans top view to make sure your nose is symmetrical and the FW is parallel to F2 (square). You can then reinforce the joint with triangle stock and optionally with glass cloth and resin. For a 55AX I would do both as that engine will make Tsunami seriously ballistic!

You could also work some carbon strips into the frame of F2 - just an idea. If you're going to use a 12 oz tank, I'd recommend fitting a cylindrical tank and have the plumbing exit in the engine mount ring. You can mount the tank to the FW via a 1/16" ply ring fronted with some dense foam. Finally, I'd also recommend fuel proofing the tank area before closing it up. It's a pain to do afterward as I discovered.

Hope this helps.

David.