75 L48. Engine has not run for at least 20 years maybe 25. I removed the spark plugs and put some mystery oil in all of the cylinders. I removed the distributor cap and was able to rotate the engine a little by hand pulling on the belts and the crank pulley. The distributor moved less than 180 degrees then the engine wouldn't move any more. I then put some engine oil in the cylinders and I am going to let it sit for a day and see if I can get some more movement out of it. I am afraid of breaking something. Should I be doing anything else or just keep working it like this?

Its possible that a piston on upwards travel hit a patch of rust...I say upward as you are unlikely to have rust on the cylinder walls that are below the pistons when stored. But after thinking about this a bit...its quite possible you have a gummed up valve stuck open. Now if you were only using the drive belts to turn the engine you really are not using any kind of force. You might try reversing the direction of the engine and then come back around forward once more but this time with a wrench on the bottom pulley. Bring it up slow, its ok to use a little muscle, you won't bend a valve just pulling along with a hand ratchet. See if you can work it past that "stuck" section...but don't force it. If you have a valve that is stuck open you will find a push rod (or 2 or 3 etc.) that is sloppy/ loose some of the rotation. You can ascertain that by removing the valve covers.
Please post results. Dave

I am going to give it some more effort today. I put another shot of oil down the cylinders yesterday so it could sit over night. I did go in the other direction with the rotation and it stopped at less than 180 degrees at that end too. I might go and get some of that gun lubricant as suggested. I'll keep progress posted. This is only the beginning.
Thanks
Jim

Use some PB blaster and try to use the spray extension to spray the walls down by directing the discharge tip in the spark plug home. 

Could always put a new crate 350 ( you can pick your HP..290,300 and more) in her, you'll love the performance boost and you won't have to worry about an old motors longevity 

Be careful....it might not necessarily be rust, or corrosion stopping the pistons/crank. You may have an old rat's nest on top of one(or more) of the pistons, if the car sat that long. They can(and will) crawl up thru the exhaust, and get into the cylinder thru opened valves. Seen it more than once.
A borescope would be perfect for looking down into the cylinders to have a look-see...if you can borrow one.

I did use some PB Blaster in addition to the oil. No luck so far. I tried to turn it this morning but I have yet to use a wrench on it. I have a camera probe that is used on the inside of walls but the probe is not small enough to fit into the spark plug hole. I removed the valve covers but I don't know which if any valve is stuck. If I have no luck after a few more tries what is the next step? Pull the heads? If the valves are stuck can I get some lube down there to free them up or do I need to remove the heads?

Got it to free up. I tried with the wrench in the direction of engine rotation but it was really stuck and I didn't want to break anything. I backed it off and went in the other direction. Back and forth several times just pulling on the belts and it went all the way around freely. I put some more 10w-30 into all of the spark plug holes. There was oil coming up into the rocker arms so it appears the oil pump is working. I need to get that old oil out of there as soon as I get the car up on some stands.
Got most of the old gas out of the tank. Next step is get underneath and try to drop the tank and replace fuel/brake lines.
It will be a while before I get it up and running. Someone suggested that I drop a new crate motor in there. I know that it is a good way to go but I am someone who really likes things original. As far as I know the car is matching numbers but I have to go through it and verify that. I know this isn't a real high value C3 but the price was right (free) and I don't have any intention of selling it. I want to restore it pretty much how it came from GM with maybe some mild internal engine mods. 

Awesome! If you were able to do this by just pulling the belts I think your in the clear. Maybe a tinge of a rust ring from an old iron compression ring in a cylinder where the valve (s) was open when stored.
Ok no one give me crap but....I've had old V8's that sat for years and replaced the old oil with trans fluid. Ran them for a few minutes and dumped that. Yes they smoked but it stopped as soon as I replaced the trans fluid with motor oil. The trans fluid does wonders to penetrate and dissolve sludge, varnish etc. A good way to loosen/clean out an old engine without breaking it down. You may want to pull the oil pan anyway just to clean it out. If the PO ran paraffin based oil (penzoil, quakerstate etc.) you might find a couple pounds of filthy wax in you pan. I've seen it so deep there were paths run through the wax for the counter weights to swing through...true for real.