The last 4 carpets I bought all came from ACC (Automotive Custom Carpets), even if they were ordered from one of the Pontiac restoration suppliers, "stockinteriors.com", JCWhitney or directly from ACC..
I always order the heavier backing and also recommend using a thermal/noise barrier like geeteeohguy recommended. On my 68 GTO, I used a roofing product from Home Depot or Lowes (don't remember where I bought it). It comes in 4" or 9" wide rolls and was easy to apply. It really helps block heat coming through the firewall & floor. Dynamat is another great choice, but it's much more expensive.
Excellent advice about comparing old to new and measuring multiple times before cutting!
It also helps to let the carpet "relax" in the hot sun before installation.
1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base