The idle speed adjustment screw is located under a black plastic blanking plug on the rear top engine side of the fuel injection throttle body housing. Make sure that the ignition timing of the HDE engine is correctly set before you set the idle speed. The timing on mine had been incorrectly set - marks MUST be aligned with the first vacuum advance (closest to the distributor- also known as "sub timing advance") disconnected and plugged.
Hope this helps.