The correct thread is 1/2" NPT, not BSP. I stock these as Raceworks part no. RWF-932-08BK, it is an Allen ket grub screw style plug.
A BSP plug is close, but not close enough, I've seen housings split open from people over tightening a BSP plug into those holes, attempting to stop the leak.
Also to block off those manifold hose ports, use an early Commodore bypass hose Mackay part no. CH1170. This hose is U-shaped & works well to act as a bypass hose.
Dr Terry