Thanks for the help. PHS says block code is YS.
The engine unit number is not fully legible. See pic. It looks like the number was never stamped deeply into the metal (or it was somehow wiped away, but the paint does not appear to be thick. The area just looks very smooth.
I have read about others in this forum who have the same missing '9' with the same engine casting number that I have: "778789".
My plan to inspect the block from underside is to pull the oild pan, then push the pistons up and out of the block to inspect the cylinder walls, rings and pistons. Your thoughts about doing a full inspection in light of the info from the previous owner is appropriate, and I am just looking for an alternative to pulling the entire motor.
Assuming the block and pistons check out OK, should I rebuild the heads, or get new heads like Edelbrock's, or look for a matched rebuilt set?
If the block is bad, then I might as well get a new engine. If it is just bad rings or pistons, then keeping the original block in tact makes sense.
Looking at the photo, someone has messed with the engine code and restamped it "YS". So with the 092 heads, it is most likely an engine out of a full size car.
To better answer your question, you need to know what kind of a budget you plan on spending and how much HP at what RPM band. Pontiac's are built for torque.
You can't put Edelbrock heads or any large valve head on a 389. The later 1967 and up valves are angled differently and repositioned more to center. So piston valve reliefs will not match and the valves may hit the cylinder wall if not notched.
You can bore a 389 to that of a 400CI and then go from there with 400CI top end if you want Edelbrock heads or big valve heads. Or you can stick with the 389 and keep the 092 heads and use that as your base.
Many options and ways to build your engine. If you do a search using the upper right hand "Google Custom Search" and type in 389 build, 389 engine, 389 to 400, 092/093 heads, etc., you will pull up past posts from this site that should give you plenty of ideas. Again: budget, power range, and parts matching to make it all work. Keep in mind that as you build bigger HP/TQ, that the rest of your driveline may not handle it and things behind the engine will break or need upgrading and that adds $$$. And as cij911
stated, you really want to pull the engine as the pan typically does not come off while in the car, but if you tear off the heads, pull the distributor, pull the radiator, and undo the engine mounts, you may get lucky enough to lift the engine high enough and drop the pan without a complete removal of the engine.