Another update:
I have almost completed covering, except for touching up in a few corners. I am HUGELY disappointed to have found that I need 6 oz of weight in the tail to balance this thing. I have no idea WTH went wrong - I checked balance along the way, and it seemed to be in the ballpark. I know I went a little heavy fuelproofing the wood toward the front, but I didn't add more than an ounce or two because of it. Considering the tail moment is several times the nose moment, it would seem that I built the nose 3 to 4 x 6 oz = 18 to 24 ounces too heavy, which I most certainly did not. I know the tail wood was quite light (actually quite fragile, too), so maybe that is the root problem, and not the design.
My Saito .62 is slightly lighter than an OS .46, so the only way to solve this problem without making it nearly a half pound heavier with lead in the tail is to spend a ton of money on a tiny, lightweight engine that will make it be underpowered. I am not going to convert this to electric. I also built a set of floats, which weigh quite a bit and have lots of drag, so a smaller engine wouldn't cut it, powering it as a floatplane.
Epic fail, and I am sorely disappointed in what I have. I think I may light it on fire and throw it off of my roof. Actually, I won't do that today, but if I feel the same way about this problem in a few days, I probably will.