I think you can use the Z Pack in the earlier model transmissions, assuming you have the correct input housing and piston.
The Z Pack uses frictions and steels very close to stock thickness. The thin clutches and steels don’t handle heat nearly as well, a few back to back full throttle upshifts will warp them.
I forgot to mention the Sonnax smart shell, I use those in every 4L60 I build. There are too many cheap Beast drive shell knocks offs to take a chance on anything else.
Monster transmission did a phenomenal job of promoting themselves. They would have been better off using some of the advertising budget in customer service. They don’t build any of the stuff the sell, it’s just repackaged parts from other venders.
Reading along I see a motor swap was involved. I'm going to venture a guess the TV cable on the 700R4 get taken loose while pulling the carb and stuff. That begs the question was it adjusted it properly once you put the unit back in operation? If not then you may have found out the hard how quickly a mis-adjusted TV cable can ruin your transmission. Not happy news but maybe an indicator of what the root of the problem is. It might also have just been a wear factor.
The clutch parts inside a 700r4 are wearable items and I can't honestly say I know anyone that ever changed the fluids on a good schedule in these old squares. I'm guilty of it my own self to be honest. If it was a stock unit with no shift kit, mods or updates then it was probably on borrowed time already.
Sounds like you are at the point now of making the right decisions for a rebuild. I would look at a good premium rebuild kit with a proper servo and shift kit, an updated wider band, and then definitely add in some auxiliary cooling. Have a transmission shop you can trust do the TV cable adjustment. You do that and ensure your lockup functions are working and your gear is matched to your tire size and power delivery needs then the trans should last a long time afterwards
Reading along I see a motor swap was involved. I'm going to venture a guess the TV cable on the 700R4 get taken loose while pulling the carb and stuff. That begs the question was it adjusted it properly once you put the unit back in operation? If not then you may have found out the hard how quickly a mis-adjusted TV cable can ruin your transmission. Not happy news but maybe an indicator of what the root of the problem is.
The best place to monitor temp is the pan. That’s the best place to get a good average temp. Don’t use the line pressure tap, there is zero fluid flow, you will be basically reading the temperature of the case. The cooler line out of the trans to the radiator will be the hottest fluid, that’s basically the fluid coming out of the torque converter.
I wouldn’t worry until the temp gets over 230-240 continuously. It might get that hot if your really working the truck, but ideally it will cool back down once you start moving and the converter locks. Obviously, the lower the temp (within reason) the better.
Adding a big cooler is always a good idea. If you drive the truck in cold climates, keeping the radiator cooler is a good idea. That prevents the fluid from getting too cool.
I had the 6L80E in my 2011 Suburban go out last year during a family camping trip while pulling through the mountains in Montana. Fully rebuilt the trans with some slightly upgraded bits and added the factory tow package radiator, engine oil cooler, and external trans cooler(in addition to the radiator cooler it had before). Last week I pulled the same trailer through the Sierras in Northern California. It was 90-100 Degrees out and ~11% grades for several miles pulling through 8000ft passes. I ended up having to slow to 35-40mph floored and both the engine and trans got up to around 240 degrees before we peaked through the pass and both came back down within a minute or two. The engine comes back down to 210 and stays there unless its under a massive load. The trans runs about 100 degrees above ambient with a small external cooler and a decent load on it. Maximum load and the external cooler just couldn't keep up in those temps. Pulling back through the mountain passes in southern Oregon at midnight on Monday, it was 50 Degrees outside and the trans temp never got over 175 degrees and the engine temps were locked in at 210.
I know its a different trans, but the ideas are the same. You need to size an external cooler for the worst case scenario. If its hot outside, pulling a load, with restricted airflow due to slow speeds, you need a LOT of external cooler to dump the heat. Its even worse in the older transmissions(like the 700R4) where there are big issues with towing in OD and potentially with the torque converter unlocked. The 6L80 is designed to tow fine in OD, the 700R4 is not. It also doesn't have enough gearing or power to pull even a slight hill with the torque converter locked, which just adds to the heat. Get yourself a True-cool 40k external cooler and only tow in 3rd and it will live a long life.