Robert
Just an additional thought on the engine mount. If you decide to use a channel like I did, try to find one that is deeper than the one that I have (7/8). Ideally, the depth should be as close to the final standoff dimension that you need without having too thick a channel wall. Then you could use a short bolt through the lower flange into the firewall captive nuts, and short bolts through the upper flnge into the motor mount. The closer you get to the standoff dimension you need, the smaller the wood spacer box has to be. The goal is to avoid long bolts. Good design practice should load a bolt in only in tension and shear. whereas bending loads are detrimental. Its hard to avoid bending loads when you use long bolts, a large clamping surface helps.
Ric