I would assume the stampings A and B do not relate to the pistons, but the dimension range on the big-end bearing diameter, or something like that. I would think in terms of balance that it would be better to run the rods that weigh the same opposite each other. If you're using the old bearings you could just plastigage the rods on the crank and that should clear things up pretty fast.
Terrible things like this don't happen where I come from. Other terrible things happen instead.
Obligatory 'build thread