I have sold plenty of flooring for these and have heard no issues with either manufacture, We also have installed ALL of the flooring from goodmark and AMD on 3 different 70 cuda's we are working on, the only issue was the front edge against the lower firewall, we had to dolly that a little, The only problem that probably can arise is that if the car was ever hit from the side or at the side cowl and the box dimension from rocker to rocker is out of wack, you will see irregularities from car to car, no 2 cars were minensionaly built alike.
if your interested we do have alot of e-body metal and assorted parts in stock, We also have good rate on shipping,