[bobcowan]

Sounds like pre-ignition (detonation) from too much timing. But, is it really? Put a timing light on it, and run the rpm's up to 2,500'ish. If your timing goes above 34* or so, then that is the problem. If not, don't waste time (and money) changing distributers and ignition parts.

The engine probably does not have headers. But run it down the highway in 3rd gear (high rpm's for a few minutes. Pull over quickly, jump out, and put a infrared thermometer on the exhaust manifold - as close to the head port as possible. If one is hotter than the others, it's running lean, and that's the cause of the detonation.

Are we sure it's detonation? Could it be a noisy lifter under a load? Stock heads probably use a pedastal rocker. Maybe the engine builder just bolted it together and didn't check valve settings properly.