Are there things in there you want to clean out that won't fit through the factory drain?
Not sure where your car is in its restoration stage but this seems to be the easiest access without additional cutting of a flap or more.
Used, of course, in conjunction with access from the drivers side vent and heater holes
Yes this examples appears to suggest that someone has been here before, doing some reconstruction
but the panels and access do illustrate and access or exit for what ever your looking to flush from the area