Im sure there are multiple threads on installing coolers here but if anyone knows a good 1 offhand Id appreciate the link..... I want to get the consensus best set up on order 1st thing Monday morning.
There's three main types of coolers... stacked plate, plate and fin, and tube and fin.
The stacked plate is the most efficient (so you can run a smaller footprint, in addition to the extra cooling benefit). But they're the most expensive. I run a tiny 10,000 BTU Derale, and it works great. But if I were still towing with the Envoy, and had it to do over again, I'd get a
slightly bigger one. Maybe 12-15K (or even 20K BTU for the really hot days, which I'm sure you have a lot of in Louisiana).
Mine is plumbed to bypass the in-radiator cooler (the guy who swapped in my rebuilt trans installed it). At first, I thought he was being stupid to entirely bypass the factory cooler, but knowing what I know now about the Envoy's OEM radiator being marginal (and that's being kind), I've come around to his line of thinking (which was based more on the factory coolers failing, tbh). If you plumb it in series with the factory cooler, send it through the external cooler LAST. Otherwise, you could actually be adding heat back in, if it goes through the radiator cooler last.
The tube and fin are the cheapest, but they're the least efficient. So one that's thermally equivalent (say, 10K BTU) will be larger. The way that the fluid goes through them (180" turns) makes them more susceptible to heat, and they tend to increase fluid pressure a little bit, as well.
Where / how you mount it matters. The best place is in front of everything else, including the A/C condenser, where it can get direct airflow. If you can mount the cooler to the front bulkhead, great. But even if you have to use those zip-tie type things they give you to connect it through the fins of the A/C condenser... that's OK, too.
If you're really worried about blocking airflow to what's there now, I know Derale makes a remote mount one that has its own fan. But that takes 12v power, of course. And I probably don't even need to say that it's $$$. Use the airflow through the grille -- it should work fine with a large enough cooler.
The 'biggest one' that I know of is made by Tru-Cool -- it's a 40,000 BTU cooler. But I don't know if it would fit in the front of the Envoy without some serious 'surgery', tbh. They're intended more for motorized RVs and such, but they'll fit in the front of a full-size pickup, too.
Whichever one you put in, make sure you put enough extra fluid in to compensate for the volume of the cooler. Some manufacturers will give you the specs on what each cooler holds, but if you can't find it, a few minutes with a quart or two of fluid, the new cooler, and a drain pan will let you know exactly how much more you'll need, prior to installing it.
Finally, the radiator fan -- I replaced my OEM with a traditional thermal fan clutch, since the OEMs always fail, and GM went back to the fan clutches at the end of the GMT360 run, anyway. But this does require the PCM to be modified, as Moose mentioned, so the truck doesn't throw a million fan codes, potentially put itself in 'reduced power mode' because it thinks it's overheating, etc.
The thermal clutches can fail, too -- but they're generally more robust than the OEM E-V type. And easier to replace (and easier to find, too).
The other thing I did when I was towing with the Envoy was to run Torque Pro on my phone, so I could display the trans fluid temp in real-time (the trans has a temp sensor, and the PCM will report the output via the OBD-II port). Even though I had bypassed the stock cooler, you could see that as the engine coolant rose in temp, so did the trans fluid temp (the lesson here is that high ambient temps and high load conditions on a driveline do take their toll.) I have a thread from a few years ago that describes the experience (towing +95F @ 6800ft, with full black / grey tanks, at 40mph, in Montana). I had to eventually pull the truck over on a grade and let her cool off for 20-30min. But I did make it up the grade without blowing a head gasket or grenading the transmission.
Even before you get a cooler, a fan clutch mod, etc. -- I'd hook up Torque Pro and see what your trans temps are normally running. That gives you a baseline to work from. With no trailer, your temps should be on the low side of 200F. If they're higher (remembering the ambient temps you get out there), size up your cooler accordingly.
For fluid, you should run Dexron VI, which will be stable to about 265F. I use Valvoline's version, but the GM is fine, too. Try not to use the 'universal' type fluids that are 'Dexron compatible'. If your trans temp gets as hot as 230F, pull over. You can monitor the temps with the engine running, which should allow them to drop. If they don't, then shut off the engine and sit for awhile. You can still monitor the temps through OBD-II, without having to start the engine (but it'll take longer to drop; they may even rise slightly at the outset, with the engine off). What you want to avoid is 'thermal runaway' (yes, it's a thing).
Finally... you might want to have a conversation with your trans builder about the slippage / flaring, before installing all of these new 'toys', and get his input. NGL -- I read your description of it, and I do have some concern (but I'm the 'worrying type', too). Keep the trans in 3rd, even on flats, in hot climates.
For some 'emergency' help with the radiator coolant temps... turn off the A/C, open the windows, and set the climate control to full heat. You'll bake... but it could mean saving the trouble of a blown head gasket or a warped head (or block, since your engine is all-aluminum). I had to do this once when I first got the Envoy and I was in a construction zone with freshly laid asphalt (no towing, still had the OEM EV clutch). It saved the day.