I think I am still leaning to a timing issue, either the cam timing or the total advance is not right. What is the initial timing + what have you got left in the distributor. To setup your mechanical advance you first need to disconnect your vacuum advance line. Next you should hook up a tach and timing light and see what your timing is set at. Watch the timing marks with the light. The reading at idle is your initial advance. Rev the motor up until the distributor stops advancing and note what speed it maxes out. This is your total advance. I think most BBC like about 10* initial and about another 20*-24* in the can. Most engines will have all timing in by 2,500 to 3,000 rpm.
If that does not work, I would drop the pan, pop off the timing cover and check how the cam timing is set up with a degree wheel, alot of work but...
p-jet
