EVERYTHING rubber is suspect. Inspect it all; replace as needed.
Hoses including PS and heater in addition to radiator hoses and brake hoses. Serpentine belt, (and it's tensionioner and the various pulleys.) Engine mount cushions (good luck finding decent replacements. At least with the clamshell design, the rotted OEM mounts aren't a safety issue like the aftermarket replacements.) Trans mount. Vacuum hoses. Look closely at the tires, (check and inflate the spare, too, and make sure you have all the OEM tire-changing tools) brake caliper mount rubber O-rings and seals (I pull the calipers apart for cleaning and inspection of the internal seal and piston) body cushions, suspension bushings, CV boots, etc. Lube all suspension/driveshaft grease zerks when you're verifying the rubber boots. And while you're lubing all the suspension joints, check 'em for wear/looseness.
ALL fluids--including gasoline if it's as old as you say it is. I dump a bottle of Techron Complete Fuel System Cleaner in the tank at every oil change. You can pump the tank dry using the fuel pump when the fuel filter is disconnected. Consider a new fuel pump and filter sock when the tank is empty. Coolant, brake fluid, axle lube F and R, PS fluid. You'll be changing the trans and transfer case fluid during the rebuild; but assure that the trans cooler(s) have been flushed, flow freely, and have no remaining debris inside. SOMETIMES it's easier to just replace the cooler(s) especially if the radiator needs attention anyway. When draining coolant, be sure to screw out the knock sensor on the passenger side, and the block drain on the driver's side so you don't have trapped coolant or flush water in the water jackets. Poke the holes with a Phillips screwdriver if they don't drain--there's often a crust of sediment that has to be broken before the coolant comes out. The knock sensor goes back in with sealant on the threads, and a specific torque. While the coolant is drained, install a replacement quick-connector in the intake manifold for the heater system. The originals are infamous for corrosion, in a rare reversal of normality, the Dorman replacement is five times as good as "Genuine GM". (At least, it was when I bought mine ~20 years ago.) I would install a Prestone "Flush and Fill" kit in a heater hose so the whole cooling system can be flushed.
ALL filters, and consider installing a PS filter in the return hose.
Park brake operation--cables are often seized or so sticky that they don't release properly. Absolutely crucial on pickups that have the crappy 254mm (10") leading-trailing shoe rear brakes.
Inspect brake linings F and R. If the rotors/drums have surface rust but no heavy wear, deep scoring, use the old pads/shoes to scrub 'em clean for twenty miles and ten stops, then consider new pads/shoes as needed.
FULLY charge battery; have battery tested for CCA (and fluid level as applicable.) Have the rest of the starting/charging system tested once the vehicle runs.
Check all the usual "tune-up" items--cap, rotor, plug wires, plugs, PCV, throttle body cleaning, maybe MAF cleaning, etc.
The Vortec distributor is a pain in the asp to get stabbed properly. Consider leaving it the hell alone. I DO NOT crank the engine with the plugs out to build oil pressure. That's just needless cranking on the starter; and needless rotations of the crankshaft, cam, pistons, wrist pins, etc. Make sure you have fuel pressure, and FIRE THAT BIITCH UP. The sooner it's running, the sooner the wrist pins, cam lobes/lifter bottoms, and cylinder walls are getting splashed with oil.
It's tempting to do a preemptive strike on the lower intake gaskets; especially if the coolant is already drained. There was a time when intake gaskets were a chronic failure point; but I think all the bad ones have been changed by now. I think I'd wait until there's a few hundred miles on the engine; that way you'd have an idea if the spider fuel injector assembly needs attention at the same time.
If you have the transmission out anyway, verify that the flexplate is in good condition--cracked flexplates are VERY common on one-piece rear main engines, and broken teeth have always been something of an issue. And...having gone that far...maybe slide a new rear-main seal into the aluminum adapter behind the flexplate.
