Need advise on leaning post install

[willy]

Any of you guys that can give me some advise would be much appreciated. Bought a leaning post last year to install on FishBones.
Came out of a Contender, size and height are perfect.
There is a big issue for me though. As you can see in the first photo it appears it will install in a great spot, BUT, there is no access to that area at all, even if I remove the fuel tank cover as there is a very large and thick stringer that runs bow to stern on each side just about where the seat/feet will be bolted in.
This stringer does not make contact with the floor apparently in this area and there is a 1.5" gap between the bottom of the floor and the stringer. Plus since it is a no wood boat there also is nothing inside the stringer, sounds hollow when you tap it.
So since I cannot just use long strong screws and go straight into the stringers I decided to do the toggle type bolts.
There in lies the problem, I cannot drill through the floor and then push the toggle set up down through it and pull it back up, the stringer will prevent that.
Any advise would be a big help.
I don't think just 3/8 screws in the floor would be stout enough for my svelte figure against that seat even though there are four holes per leg.
I wish it was so.