I'd suggest before making a decision, you should spend some time on the following site that has a focus on the V6 engine:
http://www.6066gmcguy.org/The GMC V6 was probably one of the most over engineered engines ever developed, and was designed to last almost indefinitely. There are many loyal fans of this motor. The engine is big on torque, but in stock form gas mileage is not so good. There's a simple upgrade to a two barrel Holley carb that allows better tuning.
My 1960 GMC had a 305 V6 and a granny 4 speed when I bought it. It was a torque monster, but had broken valves and was anything but quick with that transmission. I wanted a long distance freeway commuter, but I still thought about keeping it stock. The decision maker was the cost to rebuild the V6. Almost 20 years ago I could only find one shop that would work on it, and the cost just to rebuild the engine would be over $2500. For less than half that amount I bought a brand new 350 Targetmaster 350 and a 350 automatic complete with all accessories. This also made it easier to upgrade to power steering, air conditioning, etc. I believe the V6 weighs about 800 lbs, making for some tough steering. If you want power steering you'll need to fabricate your own engine mounts.
If you have a low mileage original in good condition and the truck is going to be a local cruiser, I'd suggest learning more about the history of these engines before considering swapping it for anything else.
