what plug gap are you running ?
what plugs, and how old/how many miles on them ?
what colour are they (the electrode end) ? all of them look the same colour ?
how old is the dizzy cap / rotor arm ?

if you take the lid off the ECU, you'll see a chip in one corner furthest away from the loom connector - approx 1/2 inch by 1 inch. it'll have a sticker on it (leave this in place). what does it say on it ?

with regards exhaust clearance, don't forget that under load, the engine will move, so any static clearance you may have may disappear to cause the vibration