What you need to do is get a angle grinder with a sanding disc and chamfer the existing fibreglass from the crack to about 1.5" away from it and about that far from the furthest end of the crack.

Either do one side until it's wafer thin at the crack or both sides half the thickness (i'd probably just do the inside).

Then fill up the new valley with layers of matting cut to fit in the crack so starting small and then getting bigger.

When you get almost level with the old surface then make some of the layers much bigger so they go over onto the existing floor.

Probably best to add a few extra layers over that area but put the same amount over the other seat mountings so it stays level.

Before you bolt the seat down again then sand the area where the tubes fit flat (so there are no local stress points) and also the underside where there should be big washers, at least 50mm diameter and 3mm thick, thin ones won't spread the load.

This method does work, i repaired the shell on a Imp based kit car which had cracked all the way across the floor of the car from one wheel arch to the other joining up all the suspension mounting points, that was 4 years ago and theres no sign of it returning.