As long as you brace F1 (w/ the dowls) like is says in the instruction manual you should be fine. For a little assurance, go to the grocery store and get a package of bamboo skewers. They are a helluva lot stronger than a dowl, which is what the say to use. You can also add some triangle stock to the sides and bottom where F1 meets F2. And if you are feeling really froggy and aren't too worried about the weight, glass the whole motorbox!!

As far as shimming it out an inch, you can go here.. http://www.geocities.com/roger_forgues/engstandoff.html
Great product and they can be in any length that you need. 1" will be a pretty easy one. He also sells them with right thrust already cut into it.

Or you can also use some ply and make your own.