I just purchased a 1966 Delta 88 Cinv witha 425. The engine runs, but has a knocking sound. I am not sure where it is coming from, though It does not sound good. I have read that it may be a spun bearing. Could someone tell me what else it may be? (trying to gage whether or not I need a complete rebuilt)

 

Does the engine knock at idle as well as when you accelerate hard? If it knocks only when you accelerate, there is probably too much advance in the timing and/or you need to run a higher octane gas (this problem will get worse with "winter mix" gases that have additives that make the gas more volatile for easier starting in cold weather).
When I purchased my Cutlass, the 455 had a knock at idle which turned out to be a spun bearing. Hope this helps.