The gap in the mount is for clearance for the nuts. The plates were aproximatley 1/8" thick... and aproximately 1" wide. They had slotted holes so you could set the width relative to the engine you used. They bolted into the two holes on each beam on that mount. Then you set your engine on top of those....made the necessary alignments and marked the plates with the position of the holes from your engines mounting lugs.
Paul