Welcome to the site, sounds like an interesting car you have, but unfortunately Big Pontiac's of the 60's & 70's don't bring that much money, so don't expect to get rich when selling it.
There are a few exeptions such as Super Duty, Big Engine (421, 428, or 455 cars depending on the year) or Convertible or Catalina 2+2's. But with the exeption of the Ventura Package, and the Manual Transmission Clutch Linkage your car has there are not many things to ad value to it. If it were a running driving car it would help, but with the rust issues and the locked up base 389 2bbl I would not place its current value above $2000.00 and that may be on the high side without seeing Pictures of the car.
It is an unusual combination of the base drive train and the upgraded Ventura interior, I don't think they made many like that.
As for trying to free up the engine, you could take out the spark plugs, and fill up each of the cylinder with either Marvel Mystery oil, or other penetrating oil you prefer, let is soak at least overnight, then try to turn the engine over by hand by turning the harmonic Balancer bolt. If it is just the piston rings rusted to the cylinder walls you may be lucky and it may break free. If it does then turn the engine over by hand several times to make sure it is completely free, drain & refill the oil, then with the plugs still out turn it over with the starter to get any remaining oil out of the cylinders.
But I would pull the rocker covers off 1st and see what the engine looks like around the valve springs & pushrods. If it is sludged or rusted up top I would not bother trying to free it up as I suspect it needs a total rebuild.
Anouther thing how many miles are on the car? Also do you know why the car was parked? It may have been parked for a bad engine many years ago. If the engine has not turned over in 30 years the chances of just breaking it loose and having it run OK are not good in my experience. I have gotten lucky a couple of times, but even then the engine did not last long before it started using oil.
Good Luck and post some pictures if you can.
