OK...the heater is a Smiths flat heater ( Nice, works great ). The problem
began before the heater was installed. I've been behind the instrument
cluster more times than I'd like to admit. I believe the problem lies
somewhere in the power lead going from the harness to the coil +.

I've taken the Dist apart and re-soldered all the connections to make sure
there was no short inside the Dist. then I replaced the Low tension lead back to the coil etc...

My truck is a 1970 SIIA 109 NADA with Euro spec six cyl motor. It has
a Zenith Stromberg carb and the engine is out of a 78 with the smog
stuff taken off and capped.

Just more info to add to the soup!