Assuming you're sticking with a 4L60e...
In order of importance (IMO):
1) - An auxiliary trans cooler (you can decide whether to put it in series with the stocker, or bypass entirely. I have a Derale stacked plate (most efficient, and you can use a smaller size as a result). Mine is a 10-row / 10,000 BTU in bypass mode, and it's fine except for the most stressful times (running steep grades in +90F).
I will probably put in a bigger cooler in the Sierra that will be taking over tow duty from the Voy (it has a 4L80e, which could also be an option for you, if you have the V8; I don't think there's an adapter for LL8 <> 4L80). (On edit: apparently, the 60 and the 80 have the same bolt pattern for the bellhousing, so you'd only need to worry about the driveshaft(s); the 80 is longer, so you'd need to shorten the rear one for sure. For the front, I don't know how they'd match up)
Tru-Cool also makes good coolers (and they have a neat little bypass valve you can get that keeps the fluid from running through the cooler until it hits 130F (?) or so.) Good for winter, if you worry about that sort of thing.
2) - Kit that we all reference here (check Sparky's posts in particular) that has the 'orange box' TransGo kit, Sonnax pinless accumulators, hardened separator plate, new gaskets. Some kits contain the ATSC book. Some have the Vette servo (which I'd recommend, if you're wanting to go fast). The guy is on eBay, but now has his own website.
Putting this kit in goes a long way toward taking care of preventing TCC / PWM issues, as well as broken plastic accumulators, checkballs punching through the stock plate, etc. And you can put this kit in with the trans in the car (yourself, if you're mechanically inclined - I did mine.)
Get some trans assembly lube (medium tack is fine), or some Vaseline to hold checkballs in place, etc.
Zinc-coated pan bolts and aluminum case housings don't mix. If you manage to get all the bolts out intact, get some stainless to replace them. When removing them, if you notice one getting 'tighter' as you are removing it - STOP. Put some PB Blaster on the threads, and re-seat it. Then try again a little later. If you break off the one in the back corner on the passenger side, it will be almost impossible to extract it in-car. Ask me how I know.
If you want 'extra insurance' against punching through the separator plate (or especially if you're keeping the stock plate,) you can get Torlon checkballs to replace the steel ones. You can find these pretty cheap on eBay (in a set of 8, IIRC (there's one you can't get to unless you do a full rebuild, but the really important one is the 1-2 checkball - that's the one that tends to punch through).
'easy stuff' above
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'harder (& more expensive) stuff' below
The rest of this requires the trans to be dropped...you decide if / how far you want to go. With the weak points of the 4L60 addressed, it can supposedly handle 700hp, so 500 shouldn't be too much trouble. And unless you've rebuilt one in the past, or have a spare core to work on, you probably want to find a builder for the rest of this - you can discuss these things with them, as well.
At a minimum, I'd put in a hardened sunshell (Beast, Sonnax, etc.) This is a weak point that needs to be addressed for longevity, never mind 'performance'. Since it goes in the bottom of the case, you have the opportunity to go farther...such as...
I'd also (at a minimum) put in an improved 2-4 band. I use a Raybestos Hi-energy. The kevlar-coated bands *sound* great, but a lot of F-body guys have gone through them quickly and eventually steer away from them.
- h/d clutches / steels (especially the 3-4 pack, which is the weak sister here - it's susceptible to high temps (see below)). Raybestos Hi-energy or Alto 'red' are good here.
- 4L65e parts: hardened input shaft, 5 pinion f/r planetaries
Note that if you get the input shaft, which is 300mm, you have to get a compatible converter. The stock shaft / converter is 298mm (yeah, I know...2mm...but it makes a difference)
With these and the sunshell, you have a 4L65e, if that's important to you.
I don't have the 5-pin planetaries (mine is built for towing, not racing, btw). I decided the cost wasn't worth the benefit, for what I needed.
If you're getting a new converter, you'll probably want to modify the stall speed. Stock is 2800. Mine is 3200 (mild), and I think it allows too much heat buildup on steeper grades (I downshift from 3-2 a bit more than I'd like.) So this is something I might do different next time out. Talk with your builder about what you want to do.
- new pump (10 vane is fine; the 13 vane is supposed to be the real 'H/D', but I think the 10 is fine.
You'll need 12qts (minimum) of a good fluid. Personally, I like the Dexron-specific fluids, and run the Valvoline 'full synthetic' (blue bottle, $8-ish a qt retail (which I can only find around here at O'Really's), and Summit is the best price I know of on a case of 12, even figuring in shipping). Amsoil makes some great fluid (I prefer to use something I can get locally, but that's me.)
Dex 6 is pretty much synthetic anyway, and good to 275F (but no more). Do what you can to keep temps under 225F (and that means a temp gauge with that cooler you put in earlier, either an aftermarket mechanical one, or via an OBD-II compliant app (like Torque, which is what I use...and then only when I'm towing...when I'm not, I never see more than 165F, even in traffic with the A/C on full-tilt - so I don't worry about it, otherwise). If you're putting in the bung for the mechanical gauge, make sure you've also got a bung for a drain plug, if it didn't have one.
Deep / aluminum pan? That's up to you. If you have the V8, you'll have a bear of a time with a deeper pan than stock, b/c of the exhaust crossmember, and the aluminum / 'extra cooling' pans are a bit spendy, in my book. I run a stock pan, and am fine with it (except for the gorilla at GM OKC assembly who tightened all the drain plugs).
With that, you should have a good, reliable 4L60 that you can put some power in front of.
Will it be 'full race' ? Something you can take to the strip and run pass after pass, all day long? Probably not.
Will it last the rest of the truck's life, for regular / aggressive street use & towing? I'd be really surprised if it didn't.
Keep the temps cool, the fluid changed every 20K or so, especially if you're running it hard, and you'll be golden.