HEI Conversion-Sudden Death

[67carry-all]

Well, guys, counterpoint from the guy I was ordering some electrical stuff from - his opinion is that the relay to power the HEI is the ideal way to go because it assures a constant 12+V. If I'm understanding him correctly, the power from the bulkhead goes through the ignition switch, as do other power demands such as heater fan and lights. When these are active, the demand is enough that it pulls down the power available to everything running through the ignition switch, and the HEI would therefore see a reduction in voltage. Thoughts?

Since we all like pics, here's one of the tach in question.