Father-Son 1974 Duster Project

Check the big red and black wires where they plug into the big connector block at the firewall. They are known to heat up and cause an ignition miss. A friend of mine chased a slight miss in his 67 GTX, and that's where he found it. He had added a hotter ignition like MSD, so maybe your ignition upgrade is taxing the connections at the firewall like his did.