Delancy,
WOW! I had to take notes.

I'll try to answer them all, that are different than what you posted.
On the fish plates--old school. I used poster board and traced the frame, then transfered to metal and cut with the torch. Should of had a few more beers, before cutting, alittle shaky.
On the frame, it was sandblasted as well as could be, then I sprayed internal frame coating by Eastwood, inside every hole and opening while flipping and turning the frame. I did this on several occasions, to make sure it was totally coated. Finished with the paint.
On the underbelly of the body, as you said, but more like 4 coats of Monstaliner. I had extra when I painted the doghouse, so it got a couple more layers. It turned out great.
On the firewall, after patching holes it was sanded down to bare metal, no sandblasting though and painted with Rustoleum rusty metal primer. Not sure why I did that, no Zero Rust on hand and I like the primer, so. After all the Lizard Skin and Monstaliner, I spray bombed the firewall with Cygnus White. Very hard to cover black with the white.
On the doghouse, it was sandblasted then Zero Rust, then same as the firewall.
On the body, it was sandblasted on the outside by the PO. This is the only part, I wish I could do over, just so I know, what's what. It is going to the body shop to be reworked.
I'm getting ready to start on the inside of the body. It will get the full treatment, also.
I've been trying to do everything I can think of, to stop rust for awhile. As you know it will return, but I'm making it hard on it.
I highly recommend the Lizard Skin for sound deadening, you can tap any panel on the truck and get a big DUD, no ringing hollow sound. I hope the Ceramic Insulation works on the heat, the same way. We'll see.
I don't know if this was the right or wrong way to do these things, but it's what I came up with and I'm having fun along the way.