Are you sure it's a constant 13volts to the distributor? Or did you tap into the 6v line somehow?
Also, did you get the correct coil? There are two types of HEI coils, not sure the difference other that different wire colors.
Honestly, I'd go back to GM parts unless you run over 10k rpm regularly. The stock HEI setup is really quite good on its own, the only real problem with it was at really high rpm's it gets unstable. That, and by nature it' s an extremely noisy system electrically.
Another thought is a bad voltage regulator putting too many volts into the distributor. You running a voltmeter in the truck, to see the output at highway speeds?
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.