I converted my 2000 Dodge 3500 about 2 years ago. Just like you I got a great deal on it but didn't want a dually. I participate in many forums (Cummins, Dodge, campers...) and get asked about the conversion all the time. Here's a write up about the conversion. I know you aren't interested in swapping beds so just ignore that part. This covers all second generation Dodge 3500 from 1994 to 2002. The 2003 and newer have different axles and I can't help you with those.
The conversion is actually very simple. As far as the front end goes just remove the wheel spacers and install a set of rims from a 2500. You'll also need different lug nuts. I bought a set of 32 lug nuts from Ebay that were used but in perfect shape for $20. The dealer wanted $8.00 for each one. I bought four used wheels off of a Ram 2500 from Craigslist for $200.
The rear end is just as easy. Pull off the dually wheels and install the 2500 wheels and lug nuts. The only other thing that I had to do was shorten the wheel lugs because they are too long to allow the wheel center cap to fit. You could change the lugs to 2500 lugs but I just cut them with a cut-off tool and then removed the lug nuts to clean-up the treads. It only took 10 minutes and the treads are perfect. The dually axle on the Dodge( I'm not sure about Ford or Chevy) is about 4 inches wider than a 2500 axle. Most people assume it is narrower but it's actually wider because to the large offset of the dually wheels. I know 4 inches sounds like a lot but it's not noticeable. The rear wheels on a stock SRW are two inches closer together than the front wheels (one inch on each side). On my truck, the rear wheels are two inches farther apart than the front wheels. So if you look at my truck from the rear, the rear tire is sticking out one inch farther than the front.
The only other difference from a stock SRW is the size of the brake drums (see pics). It's not a functional problem but you do see more of the brake drum. Both the Dodge 2500 and 3500 have weak rear brakes. It's a common complaint among Dodge owners. Without the rotating mass of four rear tires, my rear brakes actually work very well but aren't excessive.
The harder part of the conversion is the bed. You could take off the dually fenders and fix the bed but it would take a lot of body work. If you look at the attached photo you'll see several small holes that the bolts went through, a two inch hole for a marker light, a large hole for the fuel fill and a portion of metal removed above the wheel opening. I found it much easier to replace the bed. I got very lucky and found a bed and tailgate on Craislist for $350.00. It had on very small dent and the paint was failing on the passenger's side but that wasn't a problem for me because I needed to paint it to match any way. There were just a few other things I had to do. I to cut a few inches off of the rubber fuel fill hose which was done with a razor knife. I also had to swap the fuel fill inlet (because of the large hole needed for a diesel fuel pump nozzle) but that was only 4 small screws. The last thing was to extend the rod that lowers the rear wheel. It needed to be about one inch longer. I understand this wouldn't have been necessary if the replacement bed was the same year as the original bed.
I sold all the old parts on Ebay. I actually made more than enough money to pay for my paint job and some body work. The dually fenders are a real money maker. Dodge wants a lot for them. I found a lot of people wanted them because theirs where cracked and they couldn't get a state inspection stick until they where replaced.