The only company out there who makes a quality bolt-on notch for the NBS trucks is McGaughys. It wount cut through the entire trucks frame, but it is a decent size notch. Theirs supports the oputside, as well as the underside of the frame. Might not allow you to lay completely out, but pretty close.
We offer a weld-in notch that is designed to match the contour of the NBS trucks. If you wanted to shorten one of those to fit under the bed, and keep the strength in the frame, you could do that. However that takes you away from the bolt-on notch I know.
Let me know if there's anything else I can help you out with.