yes it will be ON curve A BECAUSE IT IS PLATE.
NOT ON CURVE C. I Thought first will be on curve B.
NOW REGARDING SA 387- P3 group
ucs 66 MDMT & thickness curve - INTERSECTION POINT is below curve A. SO YES NEED IMPACT TESTING.
So your plates must be annealed? If they are not, they must be normalized and tempered or liquid quenched and tempered per SA-387....and would then be Curve C as Jano6924 pointed out.