The clack sure sounds mechanical, but I had the same exact problem once when a plug wire came loose on me, not bad enough to misfire all the time, but it wouldn't fire under load. Pushed it back on to the plug and voila!
Also, before tearing things apart, I'd check the distributor first, the frequency of the noise seems low, maybe a rotor contacting something in the cap, or the end play is off. Then I'd do a compression test.
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I built it one piece at a time and it cost me more than a dime. 
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