Gary, sorry you are feeling overwhelmed. I didn't post that link to make you feel intimidated, but to give you some ideas on a percision build. Sometimes the manuals lack some needed instructions.

As far as your engine choice, it may be a bit much. You might want to consider getting a .90 to 1.20 4 stroke. Check the size of the prop that engine is suppose to swing.

For your bolts on the firewall, post some pictures of your engine mount. Usually you epoxy the blind nuts on the back of the firewall so you don't have to get into them again. If you are saying what I think you are saying you may need to get an adapter plate to bolt up to the engine first, then bole that up to the firewall.