If you have a stock distributor, you may need to gap at .035 inches. Also, on my Cuda I eventually went with an MSD 6AL setup, MSD wires, and MSD billet distributor. Boy what a difference! Also, it lets me gap my plugs at .060.
For now, also check if you have any crossed plug wires, put silicon in the spark plug boots, and maybe see if it goes away with higher octane gas. And sometime the ignition box performs poorly if too hot, like too close to the headers or exhaust manifold.