The threads are 5/8-18 UNF, they are NOT 3/8 BSP- (which is a pipe thread, is about 1mm bigger and 19TPI).
I take a 3/8 BSP plug, nipple, or whatever you want to screw in there, machine it to 5/8" diameter then recut the thread with a 5/8 UNF dienut.
Sure- there are/were 5/8 UNF plugs and hose nipples for this manifold which were available from BMC, I just find it easier to make my own than go hunting....
PM me if you need some made.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
