1966 GMC. I fixed it...but still don't know what I fixed?

[Kentchristensen08]

Okay, let's see if anybody on here can give me some insight about a 66 GMC pickup truck .. It was in really rough shape, it turned over and started everytime and I could drive it for 2 or 3 minutes and then would lock up and die. And for the short time it would drive it ran horribly. I rebuilt everything just hoping something would work or give me some insight. The transmission, the steering column, the clutch the carburetor. Redid the wiring, new HEI distributor, new carburetor... Still no luck. Then decided to put a new intake manifold, after that it wouldn't even start. I almost gave up and just put in a 454 that was in my motorhome. With one last grasp, I went to start rebuilding the engine. The first thing I did was try to take off the harmonic balancer. It turned a little bit and then stopped and couldn't turn anymore. I grabbed a pry bar and crank down on it. It broke free with a loud pop. I thought I either just broke my engine or fixed it so I put the harmonic balancer and crankcase back on and tried to start. It fired right up and has been running great since. I've asked a few good mechanics what could have happened? They basically don't believe that that's the sequence of events or don't know what it possibly could have been. Any ideas or preferably the answer?