Steering is the biggest challenge in a V-8 conversion on one of these trucks. You'll want to choose a solution to those issues before deciding what this will take.
You can put in a modern inline motor and avoid the steering box issues, or you can do as Cletis suggests and look for one of the big GMC sixes.
Using an adapter you can run a modern automatic behind your existing old six, or one of the newer sixes without one.
Now if you are like me, you may feel you have to convert to a V-8. I'm putting a Cadillac 500 in one of mine. But its a multi-year project and I know I have a lot of work to do. If you don't have to have a V-8 consider the sixes. A lot of guys are very happy with them, and unless you have to have a screaming hot rod or use it for towing likely it will do anything you need.
