Welcome Cooper40.
First thing I would tell you is if you are going to start removing stuff take LOTS of pictures before removing anything so you know how and where stuff goes when reassembling.
Okay, as far as your siezed engine is concerned.......
How are you trying to turn the engine over? By the fan belt, the crankshaft nut, the starter????
I had a car where the water pump was locked up, not the engine. But it seemed like the engine was locked up.
It is a good idea to pull the manifolds/valve cover plates, not only to see what's going on, BUT to clean up in there and spray lube everything, too. Also, drop the oil pan and clean that out. If the engine has not been rebuilt you'd be surprised at the sludge you'll find in the pan....AND clean the oil sump pickup screen. Also, you can spray the connecting rods and crank journals (as best as you can) to get SOME lube up in there.
I would continue to soak the cylinders. Marvel Mystery Oil is good but I know a lot of people recommend transmission fluid and acetone - mixed 50/50.
I use(d) Kroil penetrating oil, too.
You need to let it soak for a week or so and then try to turn it. If it won't turn keep soaking it and letting it sit. Either try turning it over by the crankshaft nut. You get a lot more leverage. Even better than that if you remove the flywheel cover plate (under the car) and use a large screwdriver or a pry bar on the flywheel teeth (be careful not to chip any teeth) that gives you even more leverage. Or, you can tap the starter. Do not hold the starter in the engaged position just tap it. This sometimes helps loosen things up.
You can also put the car in gear and rock it back and forth to try and unlock the engine.
If you get the engine to turn watch the valves in the head to see if any are stuck. There are usually a couple that are either tight or won't go up/down.
One other comment:
Be wary of all those "Will it run" You Tube videos. From what I have seen the majority of them have some guy pull a car out of the woods (after sitting there for 50 years) and throwing a battery in it to see if it will start. REALLY BAD IDEA!!!!
There are preparation steps (some outlined above) that need to be taken in order to avoid damaging anything - BUT it looks like you are going in the right direction
Have patience and just keep at it.
The last engine I unsiezed took me 3 months - but I got it......
Here's my 49 I am working on.
Joe