99 Gas body 2500 on a 2000 Diesel 3500

Hello,
My 3500 Is mechanically awesome!! The Body however is falling apart! Its a 2000, 3500 and I found a 99 2500 Body to put on it. The 2500 is gas and the 3500 is diesel.
I'm just wondering if anyone has any tips for when I start to tackle this project.

Will everything fit properly?
What about the gauges from gas to diesel?
While I have the body off on the 3500, Should I be replacing parts that are easier to do with the body off?

The diesel 3500 has 600km on it.

Thanks Everyone!!