tlgibson,

I tried an OS .61 FX but found that it didn't fit the stock plane because of the rear needle valve. It needed a little extra length. I had another engine on hand that fit, so I just used that one. I don't see where moving the firewall by 1/4" would be any problem but I'm no expert. A .91 will be a lot of engine in that plane! My ancient Magnum .65 is plenty. How does the weight compare between the .61 and the .91? The .61 should be plenty. If the .91 is heavier, beware.

The triangle stock in the nose is just the standard affair. Glue the sides together and fill every corner with triangle stock. Do pull the sides in when building the nose. Look at the nose ring, that will give you an idea of where the sides need to be. I couldn't get things pulled in quite enough and had to add some wood inside so I could carve a nice flowing curve to meet the spinner. Remember, you're going to carve away most of the nose. Make sure you've got enough meat inside. When I carved it I did open up a couple of holes in the corners. I just filled what I could with wood and the rest with epoxy with microballoons.

The slots in the ribs are for using a wing jig. If no jig, you can ignore the slots. What I did was to run a balsa strip through the slots and then put blocks under the strip to jig the assembly to level. It worked well for me but it sounds like you've already got your jig set up.

Dave