Hey Robo, Plio-grip is a great material, and I am very familiar with most manufacturers adhesives (Kent, 3M, Fusor, Duramix, Plio-grip etc). The reason why you can get away from spot welding and adhesive use is fairly simple, the cab is not technically structural on your trucks, the cab is just that, a cabin. The vehicle (truck) relies on the heavy structural frame for strength. A unibody vehicle relies on all of the welded panels to have ultimate strength in a lapshear and peel sense, if the manufacturers only had to glue cars together they would save thousands in construction costs. The problem that the manufacturers of the vehicles and the adhesives realize is that the glue works great in some horizontal seams where peel forces aren't realized in a collision. In seams that are only subjected to lapshear forces acting on a seam they work fine, in fact better than welding. There are a few instances in today's modern structures where adhesives alone are used, an example would be on a new Mercedes rear trunk well, where it is completely bonded, but the well is a structural part itself that won't flex and is made from SRIM plastic, which is stronger than steel (3:1) pound for pound and doesn't flex. The transmission tunnel of the Olds Aurora is plastic (for strength) and is bonded in.
If you were only sectioning in a small portion of a floor pan you would be fine, but don't bond in the whole panel at the factory seams, there is not enough bonding flange. The centre section of the car is the foundation and needs ultimate strength. These cars are already subject to a lot of twist in the centre section and you could potentially de-laminate a bonded seam if the bonding flange is less than 1" wide.
In our industry, the only approved method of using adhesives is in conjunction with STRSW, unless the manufacturer allows. And that is on today's much stronger structures. Whether we like it or not, these unibodies are relatively weak and the technology was in its infancy when they were designed.
I would never recommend using adhesives to bond in a complete panel that is part of the structure of the car. Part of a 1/4 panel such as the wheelhouse to the 1/4, yes... but never the 1/4 to end panel or B pillar vertical seams. It works in some areas, just not all.
