1965 273 A body exhaust manifold to cylinder head bolt size

I know 67 and later are 5/16 . I know 65 model heads are different at intake side, so maybe they are different at exhaust side too. In either case, the end studs go into coolant passages and need sealant applied.
Metals change in size as they heat. Different metals change at different rates/amounts. The conical washers absorb the difference.