'63 Valiant V200 wagon - rare bolt issue

are you using a tapered 1/4 20 tap or a bottoming tap? By the looks of the pictures of the original bolt you posted if you can't get a tapered 1/4 20 tap in there and turn it with a 4 inch crescent wrench you can't be holding your mouth right. I've been there myself. Take a step back and put a tapered tap in there, make sure it is straight level square plumb with plenty of lube and turn it slowly with your small crescent.