It take a fair amount of metalworking knowledge to do this right. It also requires access to metalworking tools that are not commonly available.
There are a few companies out there that specialize in this type of work. They usually work on new trucks, but can be persuaded to work on older vehicles. These companies try to avoid older vehicles because there is usually a lot of rust repair and structural beefing that is necessary to do it correctly.
Crew cabs are not cheap. Expect to pay at least $10,000 for all of the sheetmetal work and frame lengthening. The custom crew cabs on new trucks can run $30,000 to $40,000 out the door.
One company that comes to mind is Alton Fabrication. I think they are in Minnesota, but I could be wrong about that. Another is Byers Autobody, in that same general area of the country.
