It was probably equipt with the toroflow GMC V6 originally. They were not considered very good engines.
The 305/351/401/478 family of V6 engines are fairly interchangeable.
Why would you want a small block V8 in that big of a truck? You would not be able to do the work you already do with the 351C V6. The V6 engines are very strong and reliable engines that produce a lot of torque at relatively low RPM's where a truck needs it the most. In a truck that big the small block would not really improve the gas mileage like in a pickup either.
But yes they can bolt in if you change the motor mounts and bellhousing to the small block version.
Ken, The detroit diesels I believe used a different bolt pattern from the light and medium duty GMC V6. But they did make the bigger V6's with the same SAE bolt pattern found on the detroits.
