[StingRay]

In the stock cab the front floor only has to hold onto the seat in a forward crash. All of the load you impart is onto the belts in tension is on designed mounting points for that purpose. When you pull forward on the upper belt at the seat top you are trying to force the front seat mountings through the floor. It was never designed for that. 69-72 trucks that had buckets actually had bracing just to support the buckets. Clearly the front floor isn't sufficient to resist the forces if it isn't sufficient even when seat belts aren't integral. The rear floor area has a cross member and huge rear cab supports, back panel and B pillars that can handle the tension loads. Tension loads are like on a rope. Compression loads that try to push through and puncture the floor are an entirely different animal. Do you want to be in that seat as it rolls down at the front and submarines through the floor? You'll get intimate with the steering column on the way by and the passenger with the dash and glove box. I could give you more advice but I'm not going to be the one that gets sued when it's interpretation and application are not as I intend. We test this stuff and it costs a lot of money to wreck a body and find out my instincts were right. Think like its a race car floor and treat your cab floor like your life depends on it because it might. Stay with the stock belt mounting and things are very simple. Integral and things are ugly. We avoid integral like the plague.

You don't want to rigidly connect your cab to your frame with the seat and belt mounts. You can install towers on the frame or cab bottom that provide resistive compression points that forces from the cab can be directed into after a small amount of deflection.

You either do this stuff way overkill or spend a huge amount of cash leaning it all out or building it up until it's just enough. Overkill is a good strategy in low volumes.