From your description it sounds like they did as you thought and installed the floor after the tank was in. But the fact that they used bolts on one side of the frame makes it seem like they were considering the tank having to be removed in the future.
Are you sure there is a tab on the font of the rails? Can the rear and side rails slide back as a u-shaped unit leaving the front bar welded in?
Can you maybe twist or roll the rear rail after you unbolt it so the tabs release?
If you bend cut or break the tabs, will the rails still be able to hold the repaired tank up when finished?