97 GT 3.8 Engine Locked Up???

A few weeks ago, some folks here helped me with a cooling problem. I finally got it fixed by replacing the bypass tube that runs into the tensioner assembly (replaced the tensioner too). Anyways. While i was having the cooling problem, the engine over heated a few times, on one occasion to the point the engine just shut down. Well about 2 or 3 weeks after i got it fixed, i got on the highway to come home from work, and when i gave it some gas to merge, it was like it lost power. The gas petal didnt do much, and the check engine light came on. I got to the shoulder and the car was still running but REALLY REALLY ROUGH. I popped the hood and watched the engine rock back and forth REALLY hard about 5 or 6 times then it quit. It never would start after that. Like it wouldnt even crank more than like 1/8 of a crank. I got it home, all the USUAL things have been checked. I had the check engine codes read, and it says "Multiple Misfires". Great. I took the car to the shop, and they checked it briefly. Said they think the engine's locked up, its not TOTALLY locked up, but its WAY too hard to crank. I'm not sure how in depth they got into it.. i know they didnt take anything apart to check. I have already gone and bought me another 97 Grand Prix in great shape, but i'd like to get this one running again so i can sell it. I am hoping someone here could give me some insight into what may be the problem and if i can fix it w/o dropping a new engine in it. Someone told me i may be able to tear the engine down to the head an pour some oil in there and move the pistons by hand or something... Hopefully someone can help me.. Thanks!!!

An old mechanics trick is to take the plugs out and spray oil into the cylinders. A light oil such as 3 and 1 or even some penetrating oil is the best, just make sure it has lubricating qualities to it. Spray each cylinder and let it sit for a day or two then try to turn the engine over without any plugs. If it frees itself up it should spin easily. If not you will probably have to remove the engine and replace it. If it does turn over, reinstall the plugs and see if it will start. If not you will need to diagnose the problem from there.

OK, admittedly, I know VERY little about the mechanics of these motors.

BUT .... being a bit of a mechanic on V8's for years, and currently building/driving/mechanicing mini-stock dirt track cars, it seems to me that these symptoms could be the result of a worn timing chain/belt, and caused by valve to piston interferance.

I doubt its the timing "belt". In my experience when a timing belt breaks, you'd lose pressure and the engine would stop running almost immedietly. When i pulled to the shoulder, the engine was still running... not for very long, but still running.