to clean carpet get a bottle of hydrogen peroxide (cheap stuff), put it in a spray bottle...spray the stains then try and blot it up from the stains...it's not a miracle worker but will remove spots that nothing else will clean

try a pressure washer

One piece carpet is easy to remove.  Actually washing it will remove that stinky can of chemicals, along with the dirt.

The carpet is thin and dries surprisingly quickly.

While it is out, you can remove the old tar mats and assess the condition of the floor pan.  '76 cars are especially prone to rust up front and the large plugs in the floor often need attention.  I had to patch both footwells on my car (and then painted the whole pan).

Tom