In looking at the "stock" firewall insulation, it was apparent to me that the factory installed the insulation prior to making all of the penetrations for the various parts that came through the firewall. So, a faithful "factory" installation would require removing most everything along the firewall, installing the insulation, then remounting everything to the firewall again.
Also, the stock insulation is not very good insulation, in my opinion, and even my cheap Ebay pad seems more effective. I wanted to use some insulation behind the kick panels and under the carpet that was better than the stock stuff. After getting a leak from the windshield I saw how the stock insulation deteriorated when subjected to moisture and did not want to see that happen again. These old squares are very leaky and I don't think it is possible to guarantee that they will never leak, no matter what measures you take. I am not looking to sell my truck at Barrett Jackson or Mecum, so I don't need to be a purist - my truck is too far gone for that.
The only way I would use stock insulation and carpet padding is if I were to immediately sell the truck when I was done. But each to his or her own.