A guy I knew locked up a freshly rebuilt 383 one time. He said it ran about 30 seconds, and just quit running. pulled the heads, and all 8 pistons were locked in the cylinders. The machine shop he had do the bore, and who sold him the pistons did too small of a bore, cast pistons to forged pistons. The shop refused to help him, said he should have caught that in the build process! I told him they should have caught it before it ever got to him.

The new machine shop had to destroy all the pistons when removing them. They did a larger overbore, and sold him a set of pistons that matched the bore. The crank had absolutely no movement.

My guess is its piston related. Gene