[68gmsee]

You didn't say if it's the original 350 (year?) but if it is, and has never been rebuilt, you've got a good engine. Most engines of that era didn't last much past 50k, 60K without some problem like worn cam, timing chain, valves, rings or lifters. The engines being made now will last way past 100k, but not then. I think it has to do with the metal alloys being used now.

I'd say do a compression check and make sure all the cylinders are within specs.

Give it a good tune up: timing, spark plugs, distributor cap and rotor, plug wires, filters,etc.)

Replace all the gaskets and seals that you can like oil pan, valve covers and change the oil.

And, as mentioned above, drive it until it won't go...