I was talking to Eric Ballard last week and asked him about his lightweight car he ran in the late 80s.
He said he removed all the double skinning from the inside, such that you couldn't tell from outside that the car had been gutted.

In his gearbox, he lightened many of the parts.  He core drilled the mainshaft (but not past 2nd gear)  He said he removed over 1.1 kg from the ring gear alone.  The gears were narrowed and drilled.  I think he said the axles were drilled too.

The end result was that he had a drag car that weighed under 450kg without driver.  And that included a roll cage and wheelie bars.  And all steel body panels.