I've got an AMC-labelled Duraspark box with the popular "TFI upgrade" parts installed. Works great. Got the Ford TFI coil for nearly nothing from a yard.
I installed a 1.6 ohm ballast resistor, a big ceramic job, bolted to the inner fender (there is no resistance wire in my home-made harness). The damned thing gets skin-peeling hot -- easy 500 degrees. I mean, peel-your-skin-off hot like the barrel of a soldering iron.
The coil seems fine, only a slight temperature rise. The Duraspark box is the same temp as the fender.