I would tend to be looking at an ignition problem as well, but not just a timing issue. The MSD box does not control the timing at all though. He says he has a rev limiter so im assuming he is using the 6AL box (make sure you do not have the 3000rpm chip installed)... The bushings and springs you mention are to control the advance curve of the distributor, highly unlikely this is the trouble on the motor he is running. I would double check to make sure you have your wires installed in the correct firing order.
Some more info on the trouble would help out alot. Does is backfire through the carb or the exhaust? A better explanation of what happens at high RPM would help as well.
hmm, the more I read his post and what he says happens, it sure sounds like a carb issue more and more. How about if you just ease into the throttle, how does it perform. If I recall you said on your webpage your using a holley 1850 carb. I beleive this is about 650 cfm, and that kind of small for a 383CI motor. You want something from the holley 4150 line, around 800cfm. To calculate cfm needed, multiply your engine size CI x maximumRPM and divide that by 3460. that will give an approx. cfm required..
Like I said though, alot more info is needed, especially specs on the motor.
Scott
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