Augusto - It most likely has to do with the quality of the replacement Duraspark II Module you are having to use. Yes - using a ballast resistor will work wonders on lowering the load on the transistor. On the typical OEM set-up for a Duraspark II, there was a ballast resistor wire of about 1.4 OHMs and the orignal coils were 2.0 or over - total resistance was way different than a TFI coil alone. The replacement module you were using most likely would have to have that sort of total resistance in order to survive with reliability.
This diagram shows a HOT ignition that uses the GM / HEI type of "four pin" module - and a later very low OHM GM coil. It works because the GM module shortens the "dwell" and regulates current to protect its grounding transistor. The coil looks very much like the TFI coil - but has less than 1/2 the OHMs. The GM coil will melt a Ford type module in a very short time - works great with the HEI four pin.