Are you triggering off the distributor or a crank trigger? If a crank trigger, then the only potential source for the spark scatter would seem to be RFI on the pickup. I had a similar problem on an engine and never did find the source...a roller lifter failed and ended the season. Good luck chasing that down.
66 Fairlane GT Silver Blue "BRUTE" Genesis 496/C4
66 Fairlane GT Nightmist Blue 428/TKO600
66 F250 Camper Special Rangoon Red 428/4-speed
2000 Lincoln LS Sport 5-speed
