you act like it's a single piece of metal... your probably looking at 4 pieces, at a minimum... cutting, grinding and fitting them... and you'd be surprised what it's like grafting in 1/8 plate into factory sheetmetal and not have it look like some cobbled together thing...... I don't do cobbled... there's no flat cut to that factory floor that wouldn't require some degree of metalwork....
your not talking about a 2' x 2' patch, we're talking nearly 6' x probably 3'.... that also requires far more support than you may think or that sh*t will be wavy as he!!... it's lost all it's strength thru it's curves.. unibody does have it's advantages.....
in reality, I had also considered going the other way with it.. remove the floor, but still have it boxed off at the height the fiberglass floor would be at.. losing cabin space... it may actually be a better height for the dog and the windows... and it would be easier to build...
but more importantly what space that opens up below and it's possible utilization...