Just to add a little extra, the old GMC V-6's, as much as I really like them, are known for poor gas mileage.
Also, not sure about the newer models, but the cab mounts on older pick-up type cabs and the larger trucks are different.
As Dave has mentioned, they're not the best Hi-way engines, especially for long periods of time at top speed and the engines are getting harder and harder to find parts for, especially new parts.
Another thing to consider is those cabs did not hold up too good in rough weather. They did not like salt. To find a good one, you would most likely have to find one outside your area.
Don't want to sound too negative, but just want to state a few facts for you.
All in all, a truck like that would be a great truck to use as is, around town. It does sound like an interesting truck.
Bill
