Cab/chassis with bed?

[TRENDZ]

I'm considering a swap. I have a '90 2wd 250 with a few mods done to it(air /water aftercooler, turbo, 19.5 wheels) The problem is the thing leaves piles of rust everywhere I park. The floor moves when i go over rough roads. Anyway, I've been collecting parts to fix my truck('86rust free cab, 93 srw bed, doors). While coming home from work, I found a 91 cab/chassis dually. I talked to the owner, and he bought the truck for the drivetain, to install in a suburban. I offered my truck as an even swap and he said he would do it, since he had no need for the body anyway.
Here are a few questions I have...
His truck also needs a cab. It has air conditioning. The rust free cab I have does not have air. Is the fire wall the same for the heater box?
Next question- The cab/chassis rear axle is narrower than a standard dually, so I know I will need to modify the inner wheel house of the bed, and make and make small flares for the outside, have any of you guys done this(pics)?
The reason I'm considering doing this, is I want the dual rear and the longer wheel base(planning a sleeper style work box between the cab and box). The air conditioning is an added bonus.

[Begle1]

Is rust that dirty stuff that gets on the bottom of your fenders when you drive over wet ground?

[Ace]

No, that woud be mud. It's a mixture of dirt and water.

I don't know about the firewall, but I'll bet they have plastic plugs for the evap lines on non-a/c trucks. If not, making your own holes would be rather trivial, considering the level of work you are considering for this project in the first place.