Okay, I'm all wrong, I've never used this kind of caliper and I went too fast, I only read the dial and therefore .75, I forgot to put 0.6 in front to make 0.675 .
So 0.675 is definitely the RIGHT diameter with OEM by pass plate.
Tracking down a 60s Ford engineer, I'm afraid is a little tricky, especially from France but, if my Pantera has overheating issues when it will be on the road again, I'll try to machine one bypass plate with a bit smaller hole, around 0.71'' to see if it's better. I even think that it must have been tried for a long time by all the specialists of the Cleveland and am curious to know the results of their tests.