The GMC V6 was designed to be a high torque, low speed engine. Massive crankshaft, low compression and heads with small valves and runners were used to achieve this goal. If you want to make it turn fast be prepared to throw major bucks into it, and expect to be disappointed. I'm not saying it can't or shouldn't be done, just know what you're getting into. But you will find it cheaper to put in a V8 than to build a V6 to do anything other than lug.
When operated as designed, and properly maintained, you can easily get 200,000 miles out of the engine. Abuse it by running wide open all day long and it will chunk rods like there's no tomorrow.
If you haven't been there, go to
http://www.6066gmcguy.org . Jolly has a wealth of info on the V6.