Lots of black smoke indicate that not all of the fuel provided is being burned. This would probably be either (A) defective ignition, or (B) defective (or wrong) carburetor.
Looking first at ignition:
Since the coil is non-original, it would be nice to know both the inductive reactance of the coil, and the capacitive reactance of the condenser. These should be opposite and equal, thus leaving only a resistive component to the impedance of the ignition system. IT IS POSSIBLE TO INSTALL A HIGH VOLTAGE COIL OF THE WRONG INDUCTANCE, AND HAVE LESS VOLTAGE AVAILABLE THAN WITH THE ORIGINAL COIL!
Looking at the carburetor:
Since the carburetor is non-original, it would be nice to know exactly which Rochester is being used. Quite possible that some previous owner mis-matched a 216 bowl and a 235 throttle body, causing a passage mis-match and a power valve always open. Also, the 216 internal venturi would be too small for the 270 (as would one from a 235).
The fact that the issue seems to be heat related, would cause me to look at the ignition; but too small a carburetor could also be the culprit.
Jon.
