I helped my cousin install a fifth wheel hitch in his 04' 2500. Also helped install one in two of my dad's trucks (F150 and F250). The main thing with any fifth wheel is checking, and rechecking your allignment and measurements.
Is the hitch a slider or static hitch?
Did your hitch come with rails?
Also are you using the install bracket kit or are you welding or both?
If you are using the install bracket kit things will be somewhat easier because it will give you the locations on your frame rails to mount the brackets, from there you drill up and install your rails (if you are using rails).
Hope this helps some. If you have other questions feel free to throw them out and I'll do my best to tell you what we did.
I have a 2500 not a 3500 but I used a B&W turnover ball set up. It installed in about 4 hours... I was really taking my time. the only cutting was to trim a corner out of the sheet metal under the box and to drill in a 3" hole for the ball in the deck of the box. It is a real sweet system. When I bought it was also the cheapest. They also have an adapter that you can use to hook up the other style of 5th wheel.
6 hours later fifth wheel in. It was absolutly horrible. Everything that could have gone wrong, went wrong the boxed frame was a bugger to work with ,but the hitch is in tonite I am going to tackle the front mount hitch.
Yeah its got a Hemi Yeah i know the cummins has more torque Yada yada yada........