First 914 Restoration, Floor boards and tar like substance Options

[tracks914]

After removing the interior of my 914 I noticed that the floor of the car was painted the original metalic gold. The floor under the seats was not painted but were still black. The floor in front of the seats looks great with no rust anywhere, not even around the edges where it meets the frame sections. It is all sound and solid even from underneath. (under the car still has the original gold paint as well)
Under the seat was different. There is rust around the seat mounts and around most of the edges I was able to pry away enough black tar like stuff to see some surface rust. There was also some rust, not much at all, in the long depressions in the floor boards. It took me about two hours to totally remove the "black stuff" from under the seat (IMG:style_emoticons/default/smash.gif) because about 90%+ had a great bond to the original steel. (no rust at all except at the edges) After sandblasting the rust that was there I found it was all surface rust and all I have to do is prime and repaint.
2 Questions for the experienced -
1- Given what I have found, would it be worth tearing out the rest of the "black stuff" in the front 2/3 of the car floor?
2 - What is that "stuff" and can I replace it with something as good or better?
In the picture below, only the dark brown colour was rust, the light brown was the bonding left from the "black stuff"
Tx
Doug