I just did the same project your doing about 2 years ago. I believe I did a write-up on it with some pictures. I was fortunate that I had very minimal rust. I wire-wheeled and sanded what little rust was there and hit it with POR 15. I then covered EVERYTHING in Dynamat. I put it on the floors, walls, back wall, doors, wall cavities on the body panels, door cavities. Everything! I even did the ceiling. I put 2 or 3 layers on the tunnel and floor by the feet and firewall. When I pull the dash, I'll put more in. I then put down the new insulation mat, then new rubber carpet. I am not a fan of carpet in a truck. I believe I have the quietest Cummins in existence (I know, bold statement). It absolutely changed the driving experience. No rattles or ringing going over bumps. Solid "thud" when you close the door.
POR-15 is an awesome product, just a little word of advice though:
Do at least 2 coats
Do not wear anything you like
Wear gloves, but it gets sticky on the gloves
Once the can is open, you can reseal it, but. don't think you'll get the can open again You'll need a can opener to poke a hole
Do not do it within 2 weeks of a formal event (It WILL GET all over you and once dry NOTHING takes it of. NOTHING!)
The project was a major PITA, but well worth it. Good luck