Hi, New here. I had the same trouble getting a rounded sump plug out of my '63 Vitesse. I had to get underneath it and came equipped with these
https://www.ebay.co.uk/itm/144585321923
Clamped on the protruding threads and turned at the first time of asking. I replaced the sump plug with a new one, but with a nut welded on it.
Paul