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I just bought a 79 bronco with a 400 engine. I drove it about 5 miles and the engine siezed. I removed the engine and stripped it down to the block with the crank and cam. I still can not turn the engine by hand ( with socket and breaker bar) is this engine garbage?
So you took the heads and front cover etc off, so you have the short block. Block, pistons/rods/crank, cam and lifters, right?
You will have to go further in disassembly to find the problem. You could have a seized rod or main bearing. Or seized piston. Or something broken that is stuck somewhere it shouldn't be.