Sounds like the secondaries are opening a lil too soon, and/or
The stall is too low, but;
The specs on that cam are 284/284/108; and .484 lift
In at 106, the Ica is 68*, and at 700ft elevation, the cylinder pressure is predicted to be ~172psi
Usually this is too high for open-chamber iron heads to run full timing without detonation. SO
More likely,
is that the ignition timing has been pulled and/or is lazy,
Furthermore, the 68* Ica makes the bottom-end nearly as soft as a carbureted 5.2Magnum.
The only cures I know of , depending on which of the 3 or 4 above-items listed are causing it, are;
> Slow the secondaries down, and/or quicken up the Accelerator pumps, or
> get a higher stall, or
> run a higher octane gas so you can put the timing back, or
> reduce the cylinder pressure
>increase the rear gear to 4 series where it shoulddabin from the get-go. This to spool up the rpm to get out of the soft-zone AND, this may allow a lil more Power-Timing before getting into detonation.
>> or some combination
Since you have them, stuff those 3.91s in there, and see how it goes.