I am currently trying to do the something similar with one. In my case I am putting a club cab and a flat deck on and using it to tow a big goose neck. Mine is getting a 5.9 cummins so I need to get the rpms down, I have not for the life of me been able to find different gear sets for that diff. I have began the search for an auxilary transmission though, either a spicer or brown-lipe with an over drive in it.

As for tires the 20's are fairly out dated, I think 22.5's are fairly common. There are some lower profile ones out there but I am not sure how the rims would work on the trucks dayton system.