You said that when you removed the distributor "it came out in one piece". What do you mean by that ? I am wondering did you drop the oil pump shaft in the pan. Did you correctly set distributor in block deep enough to engage the oil pump shaft. So when you or your dad installed the distributor, you should have had to turn the engine clockwise until the distributor dropped in place "align with oil pump shaft". If you did not correctly seat the distributor on the oil pump shaft and KEPT turning the engine over trying to start it, Then you could have locked it up due to no oil pumping through the bottom end.

 

we tried to turn it by the key after we got the gaskets in, before we replaced the distributor. It wouldn’t go, and that’s why we replaced it. It came out like nothing. We pulled the bolt out, and just pulled straight up, and the new one seated straight in. So you’re saying that we need to seat it with the oil pump shaft? The new one seated in, which I assume it was fine. We never tried turning it by key afterward, only by a breaker on the crank, and nothing. I just want to avoid taking my motor out by ANY means. That is absolutely the LAST thing I want to do.

 

After you saw the steam from the back of the engine, you shut it off, then pulled the heads ? When was the last time it turned over ? Just before you took off the heads ? Did you turn it over with the heads off ? When did you bend the three pushrods, did the engine turn then ? Did you try turning it while setting the valves ? The distributor is mute, it has nothing to do with the locked up engine. I am just wondering did it lock up after the steam incident and you did not find it until you put it back together. Did something drop into a cylinder and get stuck, stranger things that just happen. I can't remember if a blown head gasket produces steam. Blown head gaskets push water out of the overflow in MOST cases. Other cases the water is boiled by the combustion in a cylinder or two , but that's not the whole engine getting hot. I would think steam equals overheating. Overheating doesn't lead to a locked up engine right away. Knowing whether you could turn it before you took it apart Vs. after you took it apart is Key to what happened. A bearing could not lock up just sitting there, A piston can't seize just sitting there. And the starter can't spin it fast enough to lock it up. I had a distributor gear break up once. Lost oil pressure for a short time, before it finally shut off. This didn't hurt anything because there was plenty of oil on the bearings.It would take a lot of spinning to dry out the bearings.

 

Sometimes the pump shaft and the distributor line up and it just slips in. if it went in that's all that matters the distributor won't seat if you have it wrong.

If the engine won't break lose and move in either direction that eliminates most of the easier solutions to your problem. it might not be what you want to hear but a seized engine means major work.