The outside thread is 3/4 UNF and the sump must be drilled and tapped. Can be done in the car if on a hoist. Put grease on the drill and the tap to hold the swarf.
Clean the hole with a rag and thinners or throttle body cleaner. Let dry.
Apply some Loctite 262 or 271 to the outside of the adaptor and screw it right in.
When finished and adaptor and plug fitted, pour 4 L of cheap oil or your last dump of sump oil in. Drain it out and it'll bring any remaining alloy swarf

Drill size for the 3/4" unf tap is 17.5mm. 11/16" works too.
Mine has had the shank turned down to 1/2".

Edit:
Revised adaptor drawing to come. Turn from hexagonal steel bar 24mm approx.
BEFORE


AFTER

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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.