OK, This part worked a lot better than I thought it would. Since I was unable to do anything with the inside of the frame and my truck wasn't from Reno (I am really jealous, it would have been nice to find a rust free desert truck, would have saved me a lot of time!) and had a rust problem, I had to do something with it. So I bought this Rust Reformer from Rust-Oleum.
As the picture says it chemically converts rust back into metal. I was kind of iffy on how the product would work and it cost me $80 for the gallon, but I was impressed with the outcome. Here's what I did with it. Before I put those new sides on the outside of the frame I cut a big hole on each side of the frame to insert my shop vac hose. I then took my air chisel with a big round flat tip and went around the entire frame to knock any of the bigger rust off. I then inserted the shop vac hose and suck all that crap out. I have a schutz gun (Rust proofing gun) that I attached a 5 foot length of 1/8'' plastic tube. At the end I inserted a plug and then drilled several small holes in close to the end of the line to create a spray effect. I started at one end and inserted it as far as it would go and started to spray.
Then I pulled it out and put it back in several times to get a good coating on all the surfaces. I ended up putting the line in every hole I could get it into so I new I had everything inside coated well. I used about 7/8 of the gallon inside the frame.
Here's a picture of the stuff running out of some holes in the frame. It originally is a white liquid and dries on none rusted surfaces that way too which is neat because it kind of proves it is working. On the rusted surfaces it almost instantly starts to turn this black purplish color. I also noticed there were still a few chunks inside that had falling off after I had vacuumed the inside but this rust-reformer hardened into a very chip resistant material that I could only explain as an epoxy. This was even better because any other loose stuff is now stuck to the frame and won't fall off later on and expose clean metal.
As I said I was very happy with the way this stuff worked. It even sprayed very well. The only thing I can say is wear long sleeves and gloves because if you get it on you it takes for every to get off!
This is that cross tube. I sealed in the sides of it after also cleaning the inside of it so I drilled a hole and tapped it so I could coat the inside of the tube as well. I plan re-coating the inside every so often after it's on the road so this way i can just take out the bolt and spray inside.