Jim use a flat sheet of glass laid flat on the bench top for a temp firewall.Stand a plastic engine mount beam on it and tack with super glue.then you just hold the engine in position so the carb is about 1/8" off the glass and use an engine mount bolt dipped in red paint to mark the holes,drill ONE and bolt the engine to it then tack the second engine mount to the glass and do the rest.I'd rather have my short and curlies ripped out one by one than use bolts and nuts/washers etc squashing a top