[StingRay]

Look at the vehicle you take the seat from. Material shape is very significant in overall strength of the floor. More shape and depth of that shape are typically stronger. Note material thickness, gussets and reinforcements. Also consider that the floor is most likely high strength steel which our floors are not. In reality our floors will be so different that none of what you see will likely be a good guide but start there. The rockers in a perfect rust free 67-72 truck are likely your starting point. Bridging those appropriately is also likely a good path to look into. The front floor area where the cab mount bolts go through is a very robust part of the floor but a bit far forward. You need the main reinforcement very near where the front seat mounting points are.

If there is not enough room to go down towards the frame then go up into the cab with your structure. Bolted connections are not out of the question depending how they are executed in your system. You can also have some structure below the floor and some above.

There are so many other options for donor seats. I don't get why people just don't go another direction.