an approach, for what it may be worth: clean rust down to sound metal. Might be more there than photo suggests. Fiberglas overlay should be ok if no holes larger than about 1".
I larger holes, bond or pop rivet metal patches, trying to seal underside to prevent water from wicking between patch, whether of metal or 'glass, and original floor. I large patches, say wider than 4"-6" think of forming steps or ridges into them for stiffness.
If welding patches, exercise high level of care to avoid fires (how close is the gas tank, fuel lines, etc?). Hope you get the car fixed in good shape.