I have a 66 C20 and I was thinking about doing something similar. I had a '74 GMC 427 from a cabover(same block as 366) which I have since sold. It had side motor mount bosses, so it would have been a direct bolt in. My 66 has a 283 & SM420. If you have a 6, you just need to swap the motor mount to frame brackets with a V8. You can use the flywheel & clutch from a regular v8. The 427 exhaust manifolds wouldn't work because they covered the engine mount on pass side. Regular big block manifolds or headers should work. The only other issue was the water pump. The outlet was 2.5", so I would need a reducer to the radiator. You might not have these problems depending on what the motor came out of. Good luck.
