What are the platenuts for? - what are you attaching to the aft side of the firewall using the platenuts? Depends on where the doubler is installed. If the doubler is going to be on the forward side of the firewall, then yes, dimple the firewall and countersink the .063 doubler on the forward side of the FW, and use 426 rivets, flush side aft. I assume that’s what you are asking about. If you are asking about a doubler on the aft side of the firewall with the platenut forward of the firewall, then machine countersink the (.063) doubler, but not the firewall - for the platenut rivets.