Before you add the Mag Hytec cover to the diff you may want to watch Gale Banks diff cover series on Youtube. It's enlightening. It kinda looks like a stock inside profile cover that uses the stock oil fill level is the best cover. The Mag Hytec and other square inside profile covers actually run hotter and don't allow the ring gear to carry as much lube to the pinion bearings as a stock profile cover.
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Towing a 5th wheel with a SRW truck, even a 1 ton, may be challenging. I bought my 2005 SRW 2500HD 4wd that was being traded in by a guy that was replacing it with a DRW truck for his 5th wheel camper. I was in the right place at the right time when I picked up the truck so I actually met the previous owner. He said the camper drove the truck down the mountain when he went over Donner Pass driving from from Sacramento into Reno and the wind tended to push the truck and trailer around more than was reasonable on the drive from Reno into Salt Lake City. He drove a dually that belonged to another couple he and his wife were traveling with that had a similar trailer attached and it was night and day different so he traded his 2005 in.
Taller sidewalls of the 285/70R17 tires will not add to the stability of the truck. Lower profile 265/70R17 "10-ply" Load Range E tires are as tall as I'd go. That's what I have on my 2005 2500HD T800... I have 265/75R16 Load Range E tires on my 2000 C2500 T400. I tow an 18' conventional Tri-Axle flatbed trailer with both of them and they stay reasonably planted. Not as good as a dually but good enough.
A larger AUX transmission cooler isn't a bad idea. The stock T400 T800 & T900 coolers are all plumbed with the AUX cooler after the radiator loop.
More transmission fluid doesn't hurt but you don't want the pan to hang so low it gets bashed by road trash like 18wheeler treads. The one very nice thing the aftermarket pans give you is a drain plug. If you're towing a lot you can change the ATF in the pan every 2-3 oil changes... 10,000 to 15,000 miles... without dropping the pan.
A magnefine filter in the cooler return line, that you replace with every engine oil change, will keep the crud levels way down in the ATF. Since this filter has a magnet... Every time you change this filter you're discarding any fine iron filings that would otherwise make it back to the pan to get sucked up by the pump. It's also a 25 micron paper lube filter so the 110micron strainer in the pan never sees particles large enough to catch.