Hi, I believe that Al'n backed material is called Dynamat? and is quite superior for sound and heat insulation.
I plan to place the black 3mm sheet bitumen-like material on the floor pans to keep it original. I believe these cars didn't have underfelt fitted beneath the carpet originally, but I found the carpet 'sat' better ( flatter) if you glued underfelt to the back of the carpet itself. ( pretty sure I read that tip in the forum somewhere). Gives a better finish to the carpet, better heat and sound insulation and even satifies the purists (some) when they lift your carpet to inspect the floorpans to see if you have the original material or some loose underfelt lying there. As a "would-be" purist I don't like the thought of someone seeing some shiny modern material when they lift my carpet.
Cheers
Rob