In shopping for (or building) a reproduction of a 14th c. breastplate, look for one that's the correct length. Don't expect the breastplate to cover more than the bottom of your ribs or your navel. If the bottom edge is much lower than your elbows it's probably too long.
Some modern armorers sell longer breastplates, perhaps because their customers want more coverage. Avoid these. They won't look right, and they'll hinder your movement. If you want to protect more of our torso with plate, add an apron of lames, or concealed protection beneath your gypon or gambeson.