A few things.
When you replace the water pump, replace the fan clutch as well. Those two parts should be replaced as a unit (although you have to take them apart to get the fan blade assembly moved from the old parts to the new parts.
The front cover is replaced as a unit if you have leaks from either the seal or cracks in the front cover. If I have it that far apart, I'd put a good timing set in there as well. Cheap insurance if you're planning to keep the truck longer term.
About the anti-freeze. Test the pH, or just replace it and flush the system when you do the water pump. The issue isn't losing the "anti-freeze" capabilities, the ethylene glycol lasts basically forever, so an "antifreeze tester" will show it good even if it's very old. DexcoolŽ gets very acidic when the corrosion inhibitor package degrades over time. It will eat heater cores, radiators and intake manifold gaskets when it gets acidic. That's why most 1990's and early 2000's Chevrolets (not just S series) have bypassed (or leaky) heater cores. "Long life" or "extended life" antifreeze (including DexCoolŽ) last for 5 years (or 150,000 miles if you can rack that up in 5 years). It's whichever comes first.
You mentioned throttle body options in the opening post, and asked about that. Leave it alone. It's significantly oversized for the engine. There's even a lip piece on the thing to block one side at tip-in (off throttle, to about 5° to 10° opening) so it's not overly touchy off idle. There's maybe 2hp in grinding off that lip, but the down side is that it becomes a very tempremental and touchy beast just off idle.
On the exhaust side, some "shorty headers" are good, and there's a Magnaflow crossover pipe/catalytic converter assembly availble for most of them (not legal in CA, but I'd move if that was your only issue
). The Y pipe (AKA crossover pipe) is the big restriction on these engines. You can also get someone to fab up a bigger one (2.25" or 2.5" diameter is good), but that'll be a custom piece.
Using a 3 inch pipe and 3 inch universal cat on the mid pipe assembly gains a bit, although I think for your year ZR2, that bigger cat is included in the $400ish Magnaflow assembly. You'll get better performance without a lot more noise with that Magnaflow crossover pipe/cat. If you want more noise and a bit more power still, combine that with a cat-back system you like the sound of. A 3" in dual 2.5" out FlowMaster (or knockoff) with dual outlets can have a nice sound. Just make sure you have the spare tire mounted when you get it installed, or your exhaust shop might route the pipes through the space for the spare tire. Don't ask me how I know this (that wasn't me, it was actually done by the previous owner of mine).