OK, that's a lot of years w/o much mileage... So I'm guessing it sat a LOT? It may have also run for hours at idle...
So I'd like to know what the engine looks like under the valve covers? What does it sound like cold? Hot?
Any weeps, drips, leaks?
First I'd replace the PCV valve and any hardened hoses on that circuit. Check for vacuum leaks.
Then I'd do a few short oil changes. I'd use an oversized filter (check with local Napa Auto, they'll cross reference) and I'd likely run a can of BG-109 or Kreen into it for a day before each change.
Add a magnetic drain plug and check if you are getting bits of anything at each change.
Do a solid tune-up and maybe loose the old plug wires for some modern spiral wound wires.
Does it have a ECM controlled HEI and a Quadrajet?
For oil, I'd use Valvoline Maxlife 10W-30 (red bottle blend) with the added seal conditioners. You have gaskets and seals that may be getting hard due to age, not miles...
But at that age, you should be aware that things are happening. I'd have a good brake inspection and a brake fluid change. It tends to accumulate water over time and the calipers or wheel cylinders may be going bad just from age and corrosion inside.
I'd also do a tranny service. New filter and fluid (as much as it will take on a pan-drop).
Definitely new anti-freeze and we have no idea where the pH is on the existing stuff...