Curtis. .
Upon looking to do the reverse a awhile ago I went checking into what I needed to accomplish it. Here is what I found out thru my local stealership and my buddies salvage yard with a Hollander crash ebook...
Both Dana rears are the same(front and back as far as housings go)BUT,here is where the difference lies. The front of the 3500 uses a different hub,rotor and brake assembly(offset) as compared to the 2500 which uses the standard. They,the parts of course are compatible and exchangable between the models. On the rear Dana 80 there is not much difference in the trucks,same springs,helpers,u-bolts,brake drums etc,EXCEPT,the rear wheel studs which are the different. On the 3500 they have a much longer rear stud to accept the extra wheel and through the dealer as I can already tell you they are not cheap.
Easiest way to do the conversion is the way you are looking to do it. Find someone to swap with and all you'll need to do is change the front brake,rotor and hub assemblies. You may want to get a set of inner axle seals in case you accidently pull one out to far when removing the hub the rest of the work and parts you have in front of you. Then for the rear there is two ways to go about it. First is order a set of the dually studs from the dealers(aftermarket studs are to soft and will not take much re-torqueing) so you can change yours over,OR,just swap the rear hubs between the trucks then all you should need is a set of rear axle seals.
I still have not ruled out a swap to do mine yet. I had put it on the shelf when a righteous looking 98 Quad 5spd 3500 4x4 came into my view a one of our shows that was for sale with only 5,000 miles on it. The gentleman was a little overpriced and then came down to an area which I felt was acceptable and I went down with money in hand and he got cold feet and did not sell it. I have given him a time table and if its not going to be sold I'll do mine after the winter weather breaks... ... ..... Andy