I chased an issue like that a couple years ago, turned out to be a bad coil. It idled fine but broke up at mid to high RPM's. On another note try running more initial timing and limit the total timing to 32. If you have an MSD distributor you can change the stop bushing, I think the smallest...
70 Dart, loaded with bondo running a mis-matched 360 with one J head and one X head, 904 trans and 8 3/4 rear with 3.91 gears and super stock springs..255/60/15 mickey drag radials...12.60 with 1.6 60 foot times at 106 mph