I would recommend using white vinegar to clean your tank. It works surprisingly well and will not harm the internals.
It would be difficult to effectively fill and soak the tank without the fuel sender in place to seal the bottom of the tank, It would be possible I suppose to make a blank-off plate with the correct hole pattern and use that as a temporary stopper.
Be gentle with the wiring for your fuel pump, the insulation can be hardened and brittle.