The oil either leaked out or it solidified into sludge.
Poking around with a stiff wire should produce some sludge I would think.
The oil pan needs to come off. Engine removal makes the most sense.
One might imagine all the freeze plugs are likely in need of replacement as well, in order to access them all the engine needs to come out.
Bringing a car back to life after it has been sitting unused for a quarter century usually means replacing and cleaning many of the components in the fuel, braking and cooling systems as well.
Did that recently to 2 of mine, air cooled VW buses not Pontiacs. First was a 1956 panel bus sitting outside since 1992 and after 1-1/2 years of my spare time back on the road in 2019. The other was a 1966 pickup sitting inside since 1994 and resurrected in 2020 after the better part of a year getting it going again. Spent about 3 thousand dollars on each one repairing and replacing various things including new tires.
Good luck.
