Your fan clutch is not pulling enough.
Have a look down a the bimetallic spring in the middle of the clutch. If it is "caked" with dirt and grease, you need to get it clean down to metal. The spring senses the rad temperature. If it's coated with grease/dirt, it will not activate the engine driven clutch fan.
If that is still not enough, your clutch may be dead. They leak over time and go bad. If the spring is coated with crud, that's likely to be the silicone fluid leaking past the seal.
If the clutch is good, you should hear it "roar" when you first start the truck in the cold. As you drive down the road (several hundred feet is enough) the roar should go away. If it doesn't "roar" on first start, it's likely gone.
If everything checks out, there is a modification you can do to the spring to make it engage at a lower temperature. It's actually an old trick most gearheads have know about since clutch fans came out: In a nutshell, you re-bend and lengthen the spring. It's not for the faint of heart though: you can ruin the clutch if you mess it up.
If you're still running the 5 blade fan, it has to go. It works fine for regular driving, but if towing (or blocking the grill with a plow) it doesn't pull enough air. You need the 9 bladed steel fan from the later years or you can bolt on a 21" Duramax composite fan.
A healthy 6.5 clutch fan assembly will keep the truck cool just fine when hauling a plow. Mine does fine with a 7.5' fisher on the front, never goes above 195F. I have to put our 9,000lb trailer ont eh back and head skyward to drive it up to 210F and even then it won't go higher unless I get really stupid with the loud pedal. Maxes out at 215F foot to the floor and towing for sky...
There are other things like a higher output water pump, but you should check the clutch fan before going any deeper in to it.
Once you get the Engine driven fan working correctly, you should drop those pushers. They actually restrict flow though the rad. The clutch fan pulls up around 10,000 CFM on a 6.5 with the 9 blade. Even the best electric tops out around 3,500-4,000. Even with two 4,000 CFM electrics, you're pushing 2,000CFM less than the clutch fan can pull when it's locked up. This means they are actually hindering flow. It's hard to wrap your head around, but it's true. They're just in the way....it's even possible your electrics are preventing the clutch from locking up because the spring on the clutch never gets to sit in a heat soak condition to engage the clutch. That's a bit of speculation on my part though, I'd have to run a few tests to say for sure.
One last thing (and you probably aren't going to want to hear this): 240F is death for a 6.5
It should never get above 215F to stay healthy. 225F+ will do bad things like crack heads, scuff the #8 piston skirts and possibly pop a head gasket. Maybe not today, maybe not tomorrow, but 240F will kill it.....
*edit*
D'oh!
Just noticed you were talking about your gasser, not your diesel. All the stuff I typed about the clutch fan and spring holds true for the gasser also. Just disregard all the diesel specific info.
