This is a big mistake and you have to pay something for it.
Carl's idea is what will work if you want original.
Try to remove what is necessary and just rotate, slide or hang parts to the side to find holes from inside. Cut a piece of the sound deadener out and patch it back. Won't hurt. Probably a lot of work, but when it's over, it's over. Study and plan some tricky ways to move the stuff out of the way. You may be surprised. If this is to traumatic, see below.
You can also determine approximately where the holes are using the bracket and some measurements provided by Drew and place the bracket at the measurements to double check, maybe just a 1"-1.5" hole saw or a square cutout will be all that is necessary at each location to expose threads. They will be covered up by the bracket. With hole saw, you will have to miss the nuts with guide drill and drill the guide drill thru both thicknesses of firewall. That means drilling off center or modify length of guide drill. Be carful not to puncture any inside cab parts. I would probably just use a drill stop and drill a bunch of small holes skin deep to create a cutout or find the threads. You will have extra thickness and that may affect hinge operation? If you want to get back to proper level, make a big cutout to clear bracket or modify bracket.
I don't believe measurements can be used to actually find the exact holes. A cutout or hole saw cutout is the answer to expose them.
The goofier ideas are:(got these from Carl)
Abandon old threads and
A. Weld studs onto firewall
B. Rivnuts
C. Weld bracket right to firewall.
D. Tilt front end.
E. Bottom line: I would do what Carl said. take out the seat and rig a pad to lay on.
I don't have any AD trucks so I'm probably off base.
