'63 Valiant V200 wagon - rare bolt issue

Sounds like it is time to measure what the old bolt really is. Calipers for the O.D. and then count the threads per inch (or use a thread pitch gauge), or just take a 1/4-20 bolt and nest the threads together side by side. If the threads nest perfectly then it is a 20 tpi pitch.
From what you say you ran a 1/4-20 tap in the hole and it worked but a bolt won't work? Something else is going on there.
A #12 screw is .216" dia.
Worst case is you use a proper size tap drill for 1/4-20, then retap the hole. That should convert the hole.