For hanging your mantel or shelf, we suggest that you use the hanger board included in the box. The type of hardware (not included) you will use for mounting your hanger board will depend on the medium your mantel or shelf will be attached to (i.e. brick, stone, drywall...). Please consult a professional for advice on the proper hardware. We suggest that pilot holes be drilled prior to inserting the appropriate hardware for final installation on the wall surface.
For mantels, put the mantel together by putting the main shelf on the floor, upside down. Next bolt on each leg with the hardware included with your mantel.

Any suggestions for an electric firebox to fit this mantel?

Electric fireplaces are usually a bit smaller. But I would recommend you go with a smaller unit and add some tiles or a surround. I doubt you'll find one that's exact.