A later model 6.5L or Duramax fan will not work on the 6.2L with a v-belt system. The 6.5L and Duramax engines use serpentine belts, which ends up running the water pump and fan in reverse, compared to our 6.2L's. You might be able to bolt a Duramax fan onto the 6.2L, but it won't do any good because it will be running backwards.
I think the stock 6.2L fan is just fine, it's the fan clutch that needs help. A bigger fan won't help at all if the fan clutch won't engage and start turning it.
Mine works great, I can hear it kick in and out when pulling a load. It also kicks in at idle during the summer time, when stopped at a stoplight with the a/c on. When I pull away from the stop, it really roars for a few moments until it cools down.
You can adjust the clutch so it will kick in sooner by very gently bending the spring located on the front of the clutch. You can also make it engage full time, if you wanted to, by popping the spring out of it's slots.
Casey