Flame cutting, saw cutting, plasma arc, (if you have one equipped to handle .375 plate) grinders equipped with cuttoff wheels are all means of cutting the coupons. However, flame cutting with a backing stip can be disatorous if not performed by a very skilled craftsman with a torch. We used a old milling machine that had tendencies to break down on occasion when I was teaching, but found that removing the backing strips was almost as easy by placing the coupon in a vice, and griding a v dead center of the backing strip in the direction of the weld, ensuring any tacks holding the backing stip in place are ground out, and then using a cold chisel to remove them driving it under the side.
Hope this helps.
Mike