The whole chassis is different, the Gruber runs on duel wheels which are wider, and has a narrower chassis to accommodate them, and the other one runs on super singles,and has a wider chassis, so they are different in the width between cross members, length, and depth of rails. The axles are also different lengths, and the suspension mounting points will also be incorrect.
The floor pan isn't interchangeable either, the only way to do it with out both parties getting up set is to swap complete kits. Also the chassis flitch plates would all be in the wrong places as one would be for a center load tandem set up, and the other for a tri set up.