To mount an engine you will need to install two 1/4" birch plywood rails in a vertical position, typically spaced 3" or 4" apart and centered on the keel line. The rails should run the full length of the engine compartment and be secured to the bulkheads in front of the compartment and behind the compartment as well as the compartment floor. Use epoxy and fiber glass cloth to reinforce the attachment. Be sure the interior of the hull is oil free and roughened up with about 180 grit sand paper to help the epoxy grip. The 3" or 4" comes from the width of the mount you use. The engine mount is secured between these rails with through bolts and 'T' nuts. I suggest you mock up the rail installation with the rails and the engine mount temporarily installed in the boat. Mark the engine mount to rail holes and drill them before you install the rails final.
Check out the Speedmaster engine mounts at www.rossisales.com. Seeing the mount will help you visualize the installation requirements.