Misfire

Ford F-100 pickup, 1974, 80k mi / 302 ci / 5.0 l / 2bbl carb
Starts and runs reasonbly well but with some reduction in power; rough idle and slight misfire at 1,000 - 2,000 rpm +/-; something's not right; spark to all cylinders ok; egr valve seems ok; this truck has Ford's "transition period" ignition (not Duraspark): distributor w/o points, igntion module, conventional coil; spark plug conductors look good; but presence of sludge-like unburned fuel mixture on spark plug threads inside heads, all 8 cylinders!, Like not enough spark in cylinders? Thinking of replacing entire ignition system with "all-in-one" distributor; what do you suggest? Thanks a million!