In this case structural integrity of the door opening is maintained as long as you do not cut the hinge pillar off of the sill. Think of the opening as a steel ring. As long as that ring is in tact (and no collision damage is present) it's not going to change. And this is coming from someone who's replaced just about everything that can be replaced in a cab.
You can cut all the cowl panels out without any worry as long as you do not cut through the door opening parts.
