I did one of these about a year ago. The adapter was two pieces. IIRC both had a recessed face about as deep as the thickness of the flange. In other words, the resulting BB width would be roughly the same as the width of the OPC shell of the bike you are considering converting. To be safe, buy the adapter first, then the BB after you have a measurement.

Also I want to warn you that some OPC frame constructions have tubes protruding into the cavity of the BB shell where they will interfere with the adapter. I had to machine pockets in my adapated to clear the tubes using an end mill belonging to my employer. So before buying anything I would pull the crank out and have a peek inside. Best of luck to you.